Add new builder subcommand and implement builder prune to prune build cache.

This patch adds a new builder subcommand, allowing to add more builder-related
commands in the future. Unfortunately `build` expects an argument so could not
be used as a subcommand.

This also implements `docker builder prune`, which is needed to prune the builder
cache manually without having to call `docker system prune`.

Today when relying on the legacy builder, users are able to prune dangling images
(used as build cache) by running `docker image prune`. This patch allows the
same usecase with buildkit.

Signed-off-by: Tibor Vass <tibor@docker.com>

cli/command/builder/cmd.go

@@ -0,0 +1,22 @@
+package builder
+
+import (
+	"github.com/spf13/cobra"
+
+	"github.com/docker/cli/cli"
+	"github.com/docker/cli/cli/command"
+)
+
+// NewBuilderCommand returns a cobra command for `builder` subcommands
+func NewBuilderCommand(dockerCli command.Cli) *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "builder",
+		Short: "Manage builds",
+		Args:  cli.NoArgs,
+		RunE:  command.ShowHelp(dockerCli.Err()),
+	}
+	cmd.AddCommand(
+		NewPruneCommand(dockerCli),
+	)
+	return cmd
+}