save the sql datasource into an env variable in the app container (#421)

docker-compose.yml

@@ -40,6 +40,11 @@ services:
       - MM_USERNAME=mmuser
       - MM_PASSWORD=mmuser_password
       - MM_DBNAME=mattermost
+
+      # use the credentials you've set above, in the format:
+      # MM_SQLSETTINGS_DATASOURCE=postgres://${MM_USERNAME}:${MM_PASSWORD}@db:5432/${MM_DBNAME}?sslmode=disable&connect_timeout=10
+      - MM_SQLSETTINGS_DATASOURCE=postgres://mmuser:mmuser_password@db:5432/mattermost?sslmode=disable&connect_timeout=10
+
       # in case your config is not in default location
       #- MM_CONFIG=/mattermost/config/config.json