Bug 1290531 - Build Docker images from custom tar contexts; r=dustin

Now that Docker image building is called from Python, we can start to
do advanced stuff with it.

With this commit, we switch from building Docker images directly from
the source directory ("the Docker way") to using our custom Docker image
build contexts.

The main advantage of this is that locally-built Docker images can now
use our custom Dockerfile syntax to include extra files in the build
context!

The code for building a Docker image from a context has been extracted
to its own standalone function. I have nefarious plans for this in the
future, such as the ability to override the FROM syntax to specify
URLs of images. This would allow us to host base images on our own
server, which removes a dependency on Docker Hub and improves
determinism, since images on Docker Hub change all the time.

MozReview-Commit-ID: 5lTdV8yEHkc

--HG--
extra : rebase_source : c374558b82d0d0302351ffbf3c82878c6663f40c

taskcluster/taskgraph/docker.py

@@ -10,6 +10,7 @@ import json
 import os
 import subprocess
 import tarfile
+import tempfile
 import urllib2
 import which
 
@@ -88,18 +89,17 @@ def build_image(name):
         raise Exception('Docker server is unresponsive. Run `docker ps` and '
                         'check that Docker is running')
 
-    args = [
-        docker_bin,
-        'build',
-        # Use --no-cache so we always get the latest package updates.
-        '--no-cache',
-        '-t', tag,
-        name,
-    ]
-
-    res = subprocess.call(args, cwd=IMAGE_DIR)
-    if res:
-        raise Exception('error building image')
+    # We obtain a context archive and build from that. Going through the
+    # archive creation is important: it normalizes things like file owners
+    # and mtimes to increase the chances that image generation is
+    # deterministic.
+    fd, context_path = tempfile.mkstemp()
+    os.close(fd)
+    try:
+        docker.create_context_tar(GECKO, image_dir, context_path, name)
+        docker.build_from_context(docker_bin, context_path, name, tag)
+    finally:
+        os.unlink(context_path)
 
     print('Successfully built %s and tagged with %s' % (name, tag))
 

taskcluster/taskgraph/util/docker.py

@@ -6,6 +6,9 @@ from __future__ import absolute_import, print_function, unicode_literals
 
 import hashlib
 import os
+import shutil
+import subprocess
+import tarfile
 import tempfile
 
 from mozpack.archive import (
@@ -120,3 +123,37 @@ def create_context_tar(topsrcdir, context_dir, out_path, prefix):
                 break
             h.update(data)
     return h.hexdigest()
+
+
+def build_from_context(docker_bin, context_path, prefix, tag=None):
+    """Build a Docker image from a context archive.
+
+    Given the path to a `docker` binary, a image build tar.gz (produced with
+    ``create_context_tar()``, a prefix in that context containing files, and
+    an optional ``tag`` for the produced image, build that Docker image.
+    """
+    d = tempfile.mkdtemp()
+    try:
+        with tarfile.open(context_path, 'r:gz') as tf:
+            tf.extractall(d)
+
+        # If we wanted to do post-processing of the Dockerfile, this is
+        # where we'd do it.
+
+        args = [
+            docker_bin,
+            'build',
+            # Use --no-cache so we always get the latest package updates.
+            '--no-cache',
+        ]
+
+        if tag:
+            args.extend(['-t', tag])
+
+        args.append('.')
+
+        res = subprocess.call(args, cwd=os.path.join(d, prefix))
+        if res:
+            raise Exception('error building image')
+    finally:
+        shutil.rmtree(d)