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.

.config/docker_example.yml

@@ -6,6 +6,7 @@
 #───┘ URL └─────────────────────────────────────────────────────
 
 # Final accessible URL seen by a user.
+# You can set url from an environment variable instead.
 url: https://example.tld/
 
 # ONCE YOU HAVE STARTED THE INSTANCE, DO NOT CHANGE THE
@@ -38,9 +39,11 @@ db:
   port: 5432
 
   # Database name
+  # You can set db from an environment variable instead.
   db: misskey
 
   # Auth
+  # You can set user and pass from environment variables instead.
   user: example-misskey-user
   pass: example-misskey-pass