Add zsh completion for 'docker-compose build --build-arg'

Signed-off-by: Steve Durrheimer <s.durrheimer@gmail.com>

contrib/completion/zsh/_docker-compose

@@ -199,6 +199,7 @@ __docker-compose_subcommand() {
         (build)
             _arguments \
                 $opts_help \
+                "*--build-arg=[Set build-time variables for one service.]:<varname>=<value>: " \
                 '--force-rm[Always remove intermediate containers.]' \
                 '--no-cache[Do not use cache when building the image.]' \
                 '--pull[Always attempt to pull a newer version of the image.]' \