Add make target to update composer.lock

Allows developers to update the engine's PHP dependencies without
requiring a local PHP environment. Assumes a codeclimate-phpmd image is
present, within which to run the composer.phar update.

Author: pbrisbin

Diff for: Makefile

@@ -1,6 +1,13 @@
-.PHONY: image
+.PHONY: image composer-update
 
 IMAGE_NAME ?= codeclimate/codeclimate-phpmd
 
 image:
-	docker build --tag codeclimate/codeclimate-phpmd .
+	docker build --tag $(IMAGE_NAME) .
+
+composer-update:
+	docker run \
+	  --rm \
+	  --volume $(PWD)/composer.json:/usr/src/app/composer.json:ro \
+	  $(IMAGE_NAME) \
+	  sh -c 'php composer.phar update && cat composer.lock' > composer.lock