docker: drop env to ini translation, use chatmail.ini directly

Remove update_ini.sh and the env-var-to-ini pipeline. The container now
has two config modes:

- Simple: set MAIL_DOMAIN in .env, container generates chatmail.ini
  with defaults via `cmdeploy init` on first start.
- Advanced: mount a custom chatmail.ini into the container; the init
  step is skipped when the file already exists.

This eliminates the fragile FORCE_REINIT_INI_FILE / INI_CMD_ARGS
machinery and the env vars that duplicated chatmail.ini settings

Also add *.ini and .env to .dockerignore so local config files
don't leak into the image.

.dockerignore

@@ -4,4 +4,6 @@ venv/
 __pycache__
 *.pyc
 *.orig
+*.ini
 .pytest_cache
+.env