Add bash completion for `docker build --target`

`--file` and `PATH` are honored if the are specified before `--target`.
Note that according to the command's syntax, PATH should be specified
after the options.

Signed-off-by: Harald Albers <github@albersweb.de>

contrib/completion/bash/docker

@@ -2439,6 +2439,7 @@ _docker_image_build() {
 		--network
 		--shm-size
 		--tag -t
+		--target
 		--ulimit
 	"
 	__docker_daemon_os_is windows && options_with_args+="
@@ -2505,6 +2506,19 @@ _docker_image_build() {
 			__docker_complete_image_repos_and_tags
 			return
 			;;
+		--target)
+			local context_pos=$( __docker_pos_first_nonflag "$( __docker_to_alternatives "$options_with_args" )" )
+			local context="${words[$context_pos]}"
+			context="${context:-.}"
+
+			local file="$( __docker_value_of_option '--file|f' )"
+			local default_file="${context%/}/Dockerfile"
+			local dockerfile="${file:-$default_file}"
+
+			local targets="$( sed -n 's/^FROM .\+ AS \(.\+\)/\1/p' "$dockerfile" 2>/dev/null )"
+			COMPREPLY=( $( compgen -W "$targets" -- "$cur" ) )
+			return
+			;;
 		$(__docker_to_extglob "$options_with_args") )
 			return
 			;;