Use an environment variable for the tag of the postgres image

[refs #2e355ae56da6]

Author: jfahrer

compose_example/.env

@@ -2,4 +2,5 @@ POSTGRES_DB=web_app_db
 POSTGRES_USER=app
 POSTGRES_PASSWORD=secret
 POSTGRES_HOST=pg
+PG_VERSION=9.6-alpine
 PROXY_UPSTREAM=webapp:9292

compose_example/docker-compose.yml

@@ -13,7 +13,7 @@ services:
       - 80:80
 
   pg:
-    image: postgres:9.6-alpine
+    image: postgres:${PG_VERSION:-9.6}
     environment:
       - POSTGRES_DB
       - POSTGRES_USER