Add a health check for the db service

[refs #87b1edef5b20]

Author: jfahrer

swarm/swarm-demo-frontend/docker-compose.yml

@@ -17,6 +17,11 @@ services:
       - pg-data:/var/lib/postgresql/data
     env_file:
       - config/db.env
+    healthcheck:
+      test: echo "SELECT * FROM requests LIMIT 1" | psql -h localhost -U $$POSTGRES_USER || exit 1
+      interval: 10s
+      retries: 2
+      start_period: 2m
 
   lb:
     image: jfahrer/swarm-demo-lb:v1