deprecate Dockerfile legacy 'ENV name value' syntax

The Dockerfile `ENV` instruction allows values to be set using either `ENV name=value`
or `ENV name value`. The latter (`ENV name value`) form can be ambiguous, for example,
the following defines a single env-variable (`ONE`) with value `"TWO= THREE=world"`,
but may have intended to be setting three env-vars:

    ENV ONE TWO= THREE=world

This format also does not allow setting multiple environment-variables in a single
`ENV` line in the Dockerfile.

Use of the `ENV name value` syntax is discouraged, and may be removed in a future
release. Users are encouraged to update their Dockerfiles to use the `ENV name=value`
syntax, for example:

    ENV ONE="" TWO="" THREE="world"

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/deprecated.md

@@ -54,6 +54,7 @@ Deprecated | [Kernel memory limit](#kernel-memory-limit)
 Deprecated | [Classic Swarm and overlay networks using external key/value stores](#classic-swarm-and-overlay-networks-using-cluster-store)      | v20.03.0   | -
 Deprecated | [Support for the legacy `~/.dockercfg` configuration file for authentication](#support-for-legacy-dockercfg-configuration-files)   | v20.03.0   | -
 Deprecated | [CLI plugins support](#cli-plugins-support)                                                                                        | v20.03.0   | -
+Deprecated | [Dockerfile legacy `ENV name value` syntax](#dockerfile-legacy-env-name-value-syntax)                                                                                        | v20.03.0   | -
 Deprecated | [Pushing and pulling with image manifest v2 schema 1](#pushing-and-pulling-with-image-manifest-v2-schema-1)                        | v19.03.0   | v20.03.0
 Removed    | [`docker engine` subcommands](#docker-engine-subcommands)                                                                          | v19.03.0   | v20.03.0
 Removed    | [Top-level `docker deploy` subcommand (experimental)](#top-level-docker-deploy-subcommand-experimental)                            | v19.03.0   | v20.03.0
@@ -129,6 +130,30 @@ for this file will be removed in a future release.
 
 CLI Plugin API is now marked as deprecated.
 
+### Dockerfile legacy `ENV name value` syntax
+
+**Deprecated in Release: v20.03.0**
+
+The Dockerfile `ENV` instruction allows values to be set using either `ENV name=value`
+or `ENV name value`. The latter (`ENV name value`) form can be ambiguous, for example,
+the following defines a single env-variable (`ONE`) with value `"TWO= THREE=world"`,
+but may have intended to be setting three env-vars:
+
+```dockerfile
+ENV ONE TWO= THREE=world
+```
+
+This format also does not allow setting multiple environment-variables in a single
+`ENV` line in the Dockerfile.
+
+Use of the `ENV name value` syntax is discouraged, and may be removed in a future
+release. Users are encouraged to update their Dockerfiles to use the `ENV name=value`
+syntax, for example:
+
+```dockerfile
+ENV ONE="" TWO="" THREE="world"
+```
+
 ### Pushing and pulling with image manifest v2 schema 1
 
 **Deprecated in Release: v19.03.0**