Support pulling the dev container image from gcr

This will be branch-relative.  If you are ont he `devel` branch then it
will pull that tag, it will try to pull the branch you are on and can be
overridden with the COMPOSE_TAG environment variable

tools/docker-compose.yml

@@ -2,9 +2,7 @@ version: '2'
 services:
   # Primary Tower Development Container
   tower:
-    build:
-      context: ..
-      dockerfile: tools/docker-compose/Dockerfile
+    image: gcr.io/ansible-tower-engineering/tower_devel:${TAG}
     ports:
       - "8080:8080"
       - "8013:8013"