add top-level "docker bake" command as alias for "docker buildx bake"

The [`docker buildx bake`][1] command has reached GA; this patch adds
a top-level `docker bake` command as alias for `docker buildx bake` to
improve discoverability and make it more convenient to use.

With this patch:

    docker --help

    Usage:  docker [OPTIONS] COMMAND

    A self-sufficient runtime for containers

    Common Commands:
      run         Create and run a new container from an image
      exec        Execute a command in a running container
      ps          List containers
      build       Build an image from a Dockerfile
      bake        Build from a file
      pull        Download an image from a registry
      push        Upload an image to a registry
      images      List images
    ...

The command is hidden if buildx is not installed;

    docker --help
    Usage:  docker [OPTIONS] COMMAND

    A self-sufficient runtime for containers

    Common Commands:
      run         Create and run a new container from an image
      exec        Execute a command in a running container
      ps          List containers
      build       Build an image from a Dockerfile
      pull        Download an image from a registry
      push        Upload an image to a registry
      images      List images
    ...

We can do some tweaking after this; currently it show an error
in situations where buildx is missing. We don't account for
"DOCKER_BUILDKIT=0", because this is a new feature that requires
buildx, and cannot be "disabled";

buildx missing;

    docker bake
    ERROR: bake requires the buildx component but it is missing or broken.
           Install the buildx component to use bake:
           https://docs.docker.com/go/buildx/

BuildKit disabled:

    DOCKER_BUILDKIT=0 docker bake
    ERROR: bake requires the buildx component but it is missing or broken.
           Install the buildx component to use bake:
           https://docs.docker.com/go/buildx/

[1]: https://www.docker.com/blog/ga-launch-docker-bake/

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

cli/command/builder/cmd.go

@@ -23,3 +23,22 @@ func NewBuilderCommand(dockerCli command.Cli) *cobra.Command {
 	)
 	return cmd
 }
+
+// NewBakeStubCommand returns a cobra command "stub" for the "bake" subcommand.
+// This command is a placeholder / stub that is dynamically replaced by an
+// alias for "docker buildx bake" if BuildKit is enabled (and the buildx plugin
+// installed).
+func NewBakeStubCommand(dockerCLI command.Streams) *cobra.Command {
+	return &cobra.Command{
+		Use:   "bake [OPTIONS] [TARGET...]",
+		Short: "Build from a file",
+		RunE:  command.ShowHelp(dockerCLI.Err()),
+		Annotations: map[string]string{
+			// We want to show this command in the "top" category in --help
+			// output, and not to be grouped under "management commands".
+			"category-top": "5",
+			"aliases":      "docker buildx bake",
+			"version":      "1.31",
+		},
+	}
+}