Add -f shorthand flag for prune --force

The docker builder prune command has a shorthand `-f` flag for `--force`:

    docker builder prune --help

    Usage:	docker builder prune

    Remove build cache

    Options:
      -a, --all                  Remove all unused build cache, not just dangling ones
          --filter filter        Provide filter values (e.g. 'until=24h')
      -f, --force                Do not prompt for confirmation
          --keep-storage bytes   Amount of disk space to keep for cache

Given that `buildx` can be used as a drop-in replacement for the native build
commands, it should match the UI, and also have a shorthand flag.

This patch also updates the flag's description to be in line with the docker commandline

With this patch applied;

    buildx prune --help
    Remove build cache

    Usage:
      buildx prune [flags]

    Flags:
      -a, --all                  Remove all unused images, not just dangling ones
          --filter filter        Provide filter values (e.g. 'until=24h')
      -f, --force                Do not prompt for confirmation
      -h, --help                 help for prune
          --keep-storage bytes   Amount of disk space to keep for cache
          --verbose              Provide a more verbose output

    Global Flags:
          --builder string   Override the configured builder instance

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

commands/prune.go

@@ -143,7 +143,7 @@ func pruneCmd(dockerCli command.Cli, rootOpts *rootOptions) *cobra.Command {
 	flags.Var(&options.filter, "filter", "Provide filter values (e.g. 'until=24h')")
 	flags.Var(&options.keepStorage, "keep-storage", "Amount of disk space to keep for cache")
 	flags.BoolVar(&options.verbose, "verbose", false, "Provide a more verbose output")
-	flags.BoolVar(&options.force, "force", false, "Skip the warning messages")
+	flags.BoolVarP(&options.force, "force", "f", false, "Do not prompt for confirmation")
 
 	return cmd
 }