Fix importing existing configuration into Docker setup

no ref

- I mistakenly didn't realise we don't import our `.env` file and instead populate the settings individually into each container
- This fixes that by just importing it all into the Ghost container which eases importing settings from existing installations
- This will result in a bit of duplication and split env vars but the rest are generally required across multiple containers so it makes sense

compose.yml

@@ -22,6 +22,9 @@ services:
     restart: always
     expose:
       - "2368"
+    # This is required to import current config when migrating
+    env_file:
+      - .env
     environment:
       NODE_ENV: production
       url: https://${DOMAIN:?DOMAIN environment variable is required}