enhance(backend): Load settings via environment variables (#14179)

* feat(backend): Load settings via environment variables

If they're not loaded from the config file.

* chore(docker): Add hints for environment variables

It supports users to know about them.

* docs(changelog): Add the description about this change

Users can notice what's changed by this PR.

* style(backend): Fix code syntax

To pass the linter.

compose_example.yml

@@ -17,6 +17,8 @@ services:
     networks:
       - internal_network
       - external_network
+    # env_file:
+    #   - .config/docker.env
     volumes:
       - ./files:/misskey/files
       - ./.config:/misskey/.config:ro