Adds ability to squash image after build

Allow built images to be squash to scratch.
Squashing does not destroy any images or layers, and preserves the
build cache.

Introduce a new CLI argument --squash to docker build
Introduce a new param to the build API endpoint `squash`

Once the build is complete, docker creates a new image loading the diffs
from each layer into a single new layer and references all the parent's
layers.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

command/image/build.go

@@ -59,6 +59,7 @@ type buildOptions struct {
 	compress       bool
 	securityOpt    []string
 	networkMode    string
+	squash         bool
 }
 
 // NewBuildCommand creates a new `docker build` command
@@ -110,6 +111,10 @@ func NewBuildCommand(dockerCli *command.DockerCli) *cobra.Command {
 
 	command.AddTrustedFlags(flags, true)
 
+	if dockerCli.HasExperimental() {
+		flags.BoolVar(&options.squash, "squash", false, "Squash newly built layers into a single new layer")
+	}
+
 	return cmd
 }
 
@@ -305,6 +310,7 @@ func runBuild(dockerCli *command.DockerCli, options buildOptions) error {
 		CacheFrom:      options.cacheFrom,
 		SecurityOpt:    options.securityOpt,
 		NetworkMode:    options.networkMode,
+		Squash:         options.squash,
 	}
 
 	response, err := dockerCli.Client().ImageBuild(ctx, body, buildOptions)