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

Makefile

@@ -9,6 +9,7 @@ NODE ?= node
 NPM_BIN ?= npm
 DEPS_SCRIPT ?= packaging/bundle/deps.py
 GIT_BRANCH ?= $(shell git rev-parse --abbrev-ref HEAD)
+COMPOSE_TAG ?= $(GIT_BRANCH)
 
 VENV_BASE ?= /venv
 SCL_PREFIX ?=
@@ -721,10 +722,10 @@ install:
 
 # Docker Compose Development environment
 docker-compose:
-	docker-compose -f tools/docker-compose.yml up --no-recreate
+	TAG=$(COMPOSE_TAG) docker-compose -f tools/docker-compose.yml up --no-recreate
 
 docker-compose-test:
-	cd tools && docker-compose run --rm --service-ports tower /bin/bash
+	cd tools && TAG=$(COMPOSE_TAG) docker-compose run --rm --service-ports tower /bin/bash
 
 MACHINE?=default
 docker-clean:

tools/docker-compose.yml

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