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Docker Image Specification v1.2.0
An Image is an ordered collection of root filesystem changes and the corresponding execution parameters for use within a container runtime. This specification outlines the format of these filesystem changes and corresponding parameters and describes how to create and use them for use with a container runtime and execution tool.
This version of the image specification was adopted starting in Docker 1.12.
Terminology
This specification uses the following terms:
- Layer
- Images are composed of layers. Each layer is a set of filesystem changes. Layers do not have configuration metadata such as environment variables or default arguments - these are properties of the image as a whole rather than any particular layer.
- Image JSON
- Each image has an associated JSON structure which describes some basic information about the image such as date created, author, and the ID of its parent image as well as execution/runtime configuration like its entry point, default arguments, CPU/memory shares, networking, and volumes. The JSON structure also references a cryptographic hash of each layer used by the image, and provides history information for those layers. This JSON is considered to be immutable, because changing it would change the computed ImageID. Changing it means creating a new derived image, instead of changing the existing image.
- Image Filesystem Changeset
- Each layer has an archive of the files which have been added, changed, or deleted relative to its parent layer. Using a layer-based or union filesystem such as AUFS, or by computing the diff from filesystem snapshots, the filesystem changeset can be used to present a series of image layers as if they were one cohesive filesystem.
- Layer DiffID
-
Layers are referenced by cryptographic hashes of their serialized
representation. This is a SHA256 digest over the tar archive used to
transport the layer, represented as a hexadecimal encoding of 256 bits, e.g.,
sha256:a9561eb1b190625c9adb5a9513e72c4dedafc1cb2d4c5236c9a6957ec7dfd5a9. Layers must be packed and unpacked reproducibly to avoid changing the layer ID, for example by using tar-split to save the tar headers. Note that the digest used as the layer ID is taken over an uncompressed version of the tar. - Layer ChainID
-
For convenience, it is sometimes useful to refer to a stack of layers
with a single identifier. This is called a
ChainID. For a single layer (or the layer at the bottom of a stack), theChainIDis equal to the layer'sDiffID. Otherwise theChainIDis given by the formula:ChainID(layerN) = SHA256hex(ChainID(layerN-1) + " " + DiffID(layerN)). - ImageID
-
Each image's ID is given by the SHA256 hash of its configuration JSON. It is
represented as a hexadecimal encoding of 256 bits, e.g.,
sha256:a9561eb1b190625c9adb5a9513e72c4dedafc1cb2d4c5236c9a6957ec7dfd5a9. Since the configuration JSON that gets hashed references hashes of each layer in the image, this formulation of the ImageID makes images content-addressable. - Tag
-
A tag serves to map a descriptive, user-given name to any single image
ID. Tag values are limited to the set of characters
[a-zA-Z0-9_.-], except they may not start with a.or-character. Tags are limited to 127 characters. - Repository
-
A collection of tags grouped under a common prefix (the name component
before
:). For example, in an image tagged with the namemy-app:3.1.4,my-appis the Repository component of the name. A repository name is made up of slash-separated name components, optionally prefixed by a DNS hostname. The hostname must comply with standard DNS rules, but may not contain_characters. If a hostname is present, it may optionally be followed by a port number in the format:8080. Name components may contain lowercase characters, digits, and separators. A separator is defined as a period, one or two underscores, or one or more dashes. A name component may not start or end with a separator.
Image JSON Description
Here is an example image JSON file:
{
"created": "2015-10-31T22:22:56.015925234Z",
"author": "Alyssa P. Hacker <alyspdev@example.com>",
"architecture": "amd64",
"os": "linux",
"config": {
"User": "alice",
"Memory": 2048,
"MemorySwap": 4096,
"CpuShares": 8,
"ExposedPorts": {
"8080/tcp": {}
},
"Env": [
"PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
"FOO=docker_is_a_really",
"BAR=great_tool_you_know"
],
"Entrypoint": [
"/bin/my-app-binary"
],
"Cmd": [
"--foreground",
"--config",
"/etc/my-app.d/default.cfg"
],
"Volumes": {
"/var/job-result-data": {},
"/var/log/my-app-logs": {},
},
"WorkingDir": "/home/alice",
},
"rootfs": {
"diff_ids": [
"sha256:c6f988f4874bb0add23a778f753c65efe992244e148a1d2ec2a8b664fb66bbd1",
"sha256:5f70bf18a086007016e948b04aed3b82103a36bea41755b6cddfaf10ace3c6ef"
],
"type": "layers"
},
"history": [
{
"created": "2015-10-31T22:22:54.690851953Z",
"created_by": "/bin/sh -c #(nop) ADD file:a3bc1e842b69636f9df5256c49c5374fb4eef1e281fe3f282c65fb853ee171c5 in /"
},
{
"created": "2015-10-31T22:22:55.613815829Z",
"created_by": "/bin/sh -c #(nop) CMD [\"sh\"]",
"empty_layer": true
}
]
}
Note that image JSON files produced by Docker don't contain formatting whitespace. It has been added to this example for clarity.
Image JSON Field Descriptions
-
created
string - ISO-8601 formatted combined date and time at which the image was created.
-
author
string - Gives the name and/or email address of the person or entity which created and is responsible for maintaining the image.
-
architecture
string -
The CPU architecture which the binaries in this image are built to run
on. Possible values include:
- 386
- amd64
- arm
-
os
string -
The name of the operating system which the image is built to run on.
Possible values include:
- darwin
- freebsd
- linux
-
config
struct -
The execution parameters which should be used as a base when running a
container using the image. This field can be
null, in which case any execution parameters should be specified at creation of the container.<h4>Container RunConfig Field Descriptions</h4> <dl> <dt> User <code>string</code> </dt> <dd> <p>The username or UID which the process in the container should run as. This acts as a default value to use when the value is not specified when creating a container.</p> <p>All of the following are valid:</p> <ul> <li><code>user</code></li> <li><code>uid</code></li> <li><code>user:group</code></li> <li><code>uid:gid</code></li> <li><code>uid:group</code></li> <li><code>user:gid</code></li> </ul> <p>If <code>group</code>/<code>gid</code> is not specified, the default group and supplementary groups of the given <code>user</code>/<code>uid</code> in <code>/etc/passwd</code> from the container are applied.</p> </dd> <dt> Memory <code>integer</code> </dt> <dd> Memory limit (in bytes). This acts as a default value to use when the value is not specified when creating a container. </dd> <dt> MemorySwap <code>integer</code> </dt> <dd> Total memory usage (memory + swap); set to <code>-1</code> to disable swap. This acts as a default value to use when the value is not specified when creating a container. </dd> <dt> CpuShares <code>integer</code> </dt> <dd> CPU shares (relative weight vs. other containers). This acts as a default value to use when the value is not specified when creating a container. </dd> <dt> ExposedPorts <code>struct</code> </dt> <dd> A set of ports to expose from a container running this image. This JSON structure value is unusual because it is a direct JSON serialization of the Go type <code>map[string]struct{}</code> and is represented in JSON as an object mapping its keys to an empty object. Here is an example:{ "8080": {}, "53/udp": {}, "2356/tcp": {} }Its keys can be in the format of: <ul> <li> <code>"port/tcp"</code> </li> <li> <code>"port/udp"</code> </li> <li> <code>"port"</code> </li> </ul> with the default protocol being <code>"tcp"</code> if not specified. These values act as defaults and are merged with any specified when creating a container. </dd> <dt> Env <code>array of strings</code> </dt> <dd> Entries are in the format of <code>VARNAME="var value"</code>. These values act as defaults and are merged with any specified when creating a container. </dd> <dt> Entrypoint <code>array of strings</code> </dt> <dd> A list of arguments to use as the command to execute when the container starts. This value acts as a default and is replaced by an entrypoint specified when creating a container. </dd> <dt> Cmd <code>array of strings</code> </dt> <dd> Default arguments to the entry point of the container. These values act as defaults and are replaced with any specified when creating a container. If an <code>Entrypoint</code> value is not specified, then the first entry of the <code>Cmd</code> array should be interpreted as the executable to run. </dd> <dt> Healthcheck <code>struct</code> </dt> <dd> A test to perform to determine whether the container is healthy. Here is an example:{ "Test": [ "CMD-SHELL", "/usr/bin/check-health localhost" ], "Interval": 30000000000, "Timeout": 10000000000, "Retries": 3 }The object has the following fields. <dl> <dt> Test <code>array of strings</code> </dt> <dd> The test to perform to check that the container is healthy. The options are: <ul> <li><code>[]</code> : inherit healthcheck from base image</li> <li><code>["NONE"]</code> : disable healthcheck</li> <li><code>["CMD", arg1, arg2, ...]</code> : exec arguments directly</li> <li><code>["CMD-SHELL", command]</code> : run command with system's default shell</li> </ul> The test command should exit with a status of 0 if the container is healthy, or with 1 if it is unhealthy. </dd> <dt> Interval <code>integer</code> </dt> <dd> Number of nanoseconds to wait between probe attempts. </dd> <dt> Timeout <code>integer</code> </dt> <dd> Number of nanoseconds to wait before considering the check to have hung. </dd> <dt> Retries <code>integer</code> <dt> <dd> The number of consecutive failures needed to consider a container as unhealthy. </dd> </dl> In each case, the field can be omitted to indicate that the value should be inherited from the base layer. These values act as defaults and are merged with any specified when creating a container. </dd> <dt> Volumes <code>struct</code> </dt> <dd> A set of directories which should be created as data volumes in a container running this image. This JSON structure value is unusual because it is a direct JSON serialization of the Go type <code>map[string]struct{}</code> and is represented in JSON as an object mapping its keys to an empty object. Here is an example:{ "/var/my-app-data/": {}, "/etc/some-config.d/": {}, }</dd> <dt> WorkingDir <code>string</code> </dt> <dd> Sets the current working directory of the entry point process in the container. This value acts as a default and is replaced by a working directory specified when creating a container. </dd> </dl> </dd> <dt> rootfs <code>struct</code> </dt> <dd> The rootfs key references the layer content addresses used by the image. This makes the image config hash depend on the filesystem hash. rootfs has two subkeys: <ul> <li> <code>type</code> is usually set to <code>layers</code>. </li> <li> <code>diff_ids</code> is an array of layer content hashes (<code>DiffIDs</code>), in order from bottom-most to top-most. </li> </ul> Here is an example rootfs section:"rootfs": { "diff_ids": [ "sha256:c6f988f4874bb0add23a778f753c65efe992244e148a1d2ec2a8b664fb66bbd1", "sha256:5f70bf18a086007016e948b04aed3b82103a36bea41755b6cddfaf10ace3c6ef", "sha256:13f53e08df5a220ab6d13c58b2bf83a59cbdc2e04d0a3f041ddf4b0ba4112d49" ], "type": "layers" }</dd> <dt> history <code>struct</code> </dt> <dd> <code>history</code> is an array of objects describing the history of each layer. The array is ordered from bottom-most layer to top-most layer. The object has the following fields. <ul> <li> <code>created</code>: Creation time, expressed as a ISO-8601 formatted combined date and time </li> <li> <code>author</code>: The author of the build point </li> <li> <code>created_by</code>: The command which created the layer </li> <li> <code>comment</code>: A custom message set when creating the layer </li> <li> <code>empty_layer</code>: This field is used to mark if the history item created a filesystem diff. It is set to true if this history item doesn't correspond to an actual layer in the rootfs section (for example, a command like ENV which results in no change to the filesystem). </li> </ul>Here is an example history section:
"history": [ { "created": "2015-10-31T22:22:54.690851953Z", "created_by": "/bin/sh -c #(nop) ADD file:a3bc1e842b69636f9df5256c49c5374fb4eef1e281fe3f282c65fb853ee171c5 in /" }, { "created": "2015-10-31T22:22:55.613815829Z", "created_by": "/bin/sh -c #(nop) CMD [\"sh\"]", "empty_layer": true } ]</dd>
Any extra fields in the Image JSON struct are considered implementation specific and should be ignored by any implementations which are unable to interpret them.
Creating an Image Filesystem Changeset
An example of creating an Image Filesystem Changeset follows.
An image root filesystem is first created as an empty directory. Here is the
initial empty directory structure for the a changeset using the
randomly-generated directory name c3167915dc9d (actual layer DiffIDs are
generated based on the content).
c3167915dc9d/
Files and directories are then created:
c3167915dc9d/
etc/
my-app-config
bin/
my-app-binary
my-app-tools
The c3167915dc9d directory is then committed as a plain Tar archive with
entries for the following files:
etc/my-app-config
bin/my-app-binary
bin/my-app-tools
To make changes to the filesystem of this container image, create a new
directory, such as f60c56784b83, and initialize it with a snapshot of the
parent image's root filesystem, so that the directory is identical to that
of c3167915dc9d. NOTE: a copy-on-write or union filesystem can make this very
efficient:
f60c56784b83/
etc/
my-app-config
bin/
my-app-binary
my-app-tools
This example change is going add a configuration directory at /etc/my-app.d
which contains a default config file. There's also a change to the
my-app-tools binary to handle the config layout change. The f60c56784b83
directory then looks like this:
f60c56784b83/
etc/
my-app.d/
default.cfg
bin/
my-app-binary
my-app-tools
This reflects the removal of /etc/my-app-config and creation of a file and
directory at /etc/my-app.d/default.cfg. /bin/my-app-tools has also been
replaced with an updated version. Before committing this directory to a
changeset, because it has a parent image, it is first compared with the
directory tree of the parent snapshot, f60c56784b83, looking for files and
directories that have been added, modified, or removed. The following changeset
is found:
Added: /etc/my-app.d/default.cfg
Modified: /bin/my-app-tools
Deleted: /etc/my-app-config
A Tar Archive is then created which contains only this changeset: The added
and modified files and directories in their entirety, and for each deleted item
an entry for an empty file at the same location but with the basename of the
deleted file or directory prefixed with .wh.. The filenames prefixed with
.wh. are known as "whiteout" files. NOTE: For this reason, it is not possible
to create an image root filesystem which contains a file or directory with a
name beginning with .wh.. The resulting Tar archive for f60c56784b83 has
the following entries:
/etc/my-app.d/default.cfg
/bin/my-app-tools
/etc/.wh.my-app-config
Any given image is likely to be composed of several of these Image Filesystem Changeset tar archives.
Combined Image JSON + Filesystem Changeset Format
There is also a format for a single archive which contains complete information about an image, including:
- repository names/tags
- image configuration JSON file
- all tar archives of each layer filesystem changesets
For example, here's what the full archive of library/busybox is (displayed in
tree format):
.
├── 47bcc53f74dc94b1920f0b34f6036096526296767650f223433fe65c35f149eb.json
├── 5f29f704785248ddb9d06b90a11b5ea36c534865e9035e4022bb2e71d4ecbb9a
│ ├── VERSION
│ ├── json
│ └── layer.tar
├── a65da33792c5187473faa80fa3e1b975acba06712852d1dea860692ccddf3198
│ ├── VERSION
│ ├── json
│ └── layer.tar
├── manifest.json
└── repositories
There is a directory for each layer in the image. Each directory is named with a 64 character hex name that is deterministically generated from the layer information. These names are not necessarily layer DiffIDs or ChainIDs. Each of these directories contains 3 files:
VERSION- The schema version of thejsonfilejson- The legacy JSON metadata for an image layer. In this version of the image specification, layers don't have JSON metadata, but in version 1, they did. A file is created for each layer in the v1 format for backward compatibility.layer.tar- The Tar archive of the filesystem changeset for an image layer.
Note that this directory layout is only important for backward compatibility.
Current implementations use the paths specified in manifest.json.
The content of the VERSION files is simply the semantic version of the JSON
metadata schema:
1.0
The repositories file is another JSON file which describes names/tags:
{
"busybox":{
"latest":"5f29f704785248ddb9d06b90a11b5ea36c534865e9035e4022bb2e71d4ecbb9a"
}
}
Every key in this object is the name of a repository, and maps to a collection
of tag suffixes. Each tag maps to the ID of the image represented by that tag.
This file is only used for backwards compatibility. Current implementations use
the manifest.json file instead.
The manifest.json file provides the image JSON for the top-level image, and
optionally for parent images that this image was derived from. It consists of
an array of metadata entries:
[
{
"Config": "47bcc53f74dc94b1920f0b34f6036096526296767650f223433fe65c35f149eb.json",
"RepoTags": ["busybox:latest"],
"Layers": [
"a65da33792c5187473faa80fa3e1b975acba06712852d1dea860692ccddf3198/layer.tar",
"5f29f704785248ddb9d06b90a11b5ea36c534865e9035e4022bb2e71d4ecbb9a/layer.tar"
]
}
]
There is an entry in the array for each image.
The Config field references another file in the tar which includes the image
JSON for this image.
The RepoTags field lists references pointing to this image.
The Layers field points to the filesystem changeset tars.
An optional Parent field references the imageID of the parent image. This
parent must be part of the same manifest.json file.
This file shouldn't be confused with the distribution manifest, used to push and pull images.
Generally, implementations that support this version of the spec will use
the manifest.json file if available, and older implementations will use the
legacy */json files and repositories.