Fix bash completion for `docker volume ls --dangling=false`.

Signed-off-by: David Calavera <david.calavera@gmail.com>

contrib/completion/bash/docker

@@ -1988,7 +1988,14 @@ _docker_volume_inspect() {
 _docker_volume_ls() {
 	case "$prev" in
 		--filter|-f)
-			COMPREPLY=( $( compgen -W "dangling=true" -- "$cur" ) )
+			COMPREPLY=( $( compgen -W "dangling" -- "$cur" ) )
+			return
+			;;
+	esac
+
+	case "${words[$cword-2]}$prev=" in
+		*dangling=*)
+			COMPREPLY=( $( compgen -W "true false" -- "${cur#=}" ) )
 			return
 			;;
 	esac