Improve CLI documentation for pack build --tag

Closes #1136

Signed-off-by: Ujjwal Goyal <[email protected]>

internal/commands/build.go

@@ -196,7 +196,7 @@ This option may set DOCKER_HOST environment variable for the build container if
 	cmd.Flags().StringVar(&buildFlags.Policy, "pull-policy", "", `Pull policy to use. Accepted values are always, never, and if-not-present. (default "always")`)
 	cmd.Flags().StringVarP(&buildFlags.Registry, "buildpack-registry", "r", cfg.DefaultRegistryName, "Buildpack Registry by name")
 	cmd.Flags().StringVar(&buildFlags.RunImage, "run-image", "", "Run image (defaults to default stack's run image)")
-	cmd.Flags().StringSliceVarP(&buildFlags.AdditionalTags, "tag", "t", nil, "Additional tags to push the output image to."+multiValueHelp("tag"))
+	cmd.Flags().StringSliceVarP(&buildFlags.AdditionalTags, "tag", "t", nil, "Additional tags to push the output image to.\nTags should be in the format 'image:tag' or 'repository/image:tag'."+multiValueHelp("tag"))
 	cmd.Flags().BoolVar(&buildFlags.TrustBuilder, "trust-builder", false, "Trust the provided builder\nAll lifecycle phases will be run in a single container (if supported by the lifecycle).")
 	cmd.Flags().StringArrayVar(&buildFlags.Volumes, "volume", nil, "Mount host volume into the build container, in the form '<host path>:<target path>[:<options>]'.\n- 'host path': Name of the volume or absolute directory path to mount.\n- 'target path': The path where the file or directory is available in the container.\n- 'options' (default \"ro\"): An optional comma separated list of mount options.\n    - \"ro\", volume contents are read-only.\n    - \"rw\", volume contents are readable and writeable.\n    - \"volume-opt=<key>=<value>\", can be specified more than once, takes a key-value pair consisting of the option name and its value."+multiValueHelp("volume"))
 	cmd.Flags().StringVar(&buildFlags.Workspace, "workspace", "", "Location at which to mount the app dir in the build image")