Use a .env file to manage environment variables

[refs #5eda42a882d4]

Author: jfahrer

compose_example/.env

@@ -0,0 +1,4 @@
+POSTGRES_DB=web_app_db
+POSTGRES_USER=app
+POSTGRES_PASSWORD=secret
+POSTGRES_HOST=pg

compose_example/docker-compose.yml

@@ -10,18 +10,22 @@ services:
 
   pg:
     image: postgres:9.6-alpine
-    env_file:
-      - ./db.env
+    environment:
+      - POSTGRES_DB
+      - POSTGRES_USER
+      - POSTGRES_PASSWORD
     volumes:
       - pg-data:/var/lib/postgresql/data
 
   webapp:
     image: jfahrer/demo_web_app:latest
     ports:
       - 9292:9292
-    env_file:
-      - ./db.env
-      - ./app.env
+    environment:
+      - POSTGRES_DB
+      - POSTGRES_USER
+      - POSTGRES_PASSWORD
+      - POSTGRES_HOST
 
   alpine:
     image: alpine:latest