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.env

@@ -1,5 +1,11 @@
+# misskey settings
+# MISSKEY_URL=https://example.tld/
+
 # db settings
 POSTGRES_PASSWORD=example-misskey-pass
+# DATABASE_PASSWORD=${POSTGRES_PASSWORD}
 POSTGRES_USER=example-misskey-user
+# DATABASE_USER=${POSTGRES_USER}
 POSTGRES_DB=misskey
+# DATABASE_DB=${POSTGRES_DB}
 DATABASE_URL="postgres://${POSTGRES_USER}:${POSTGRES_PASSWORD}@db:5432/${POSTGRES_DB}"