app manager (docker/weblate): use new files feature to set settings-override.py:

This file is used to set `USE_X_FORWARDED_HOST`, which is necessary to make Weblate work
correctly being an Apache (and potentially other HTTP servers) reverse proxy (otherwise we
have CSRF errors, cf.  https://github.com/WeblateOrg/weblate/issues/1912).

sat/plugins/plugin_app_manager_docker/sat_app_weblate.yaml

@@ -1,6 +1,10 @@
 type: docker-compose
 prepare:
   git: https://github.com/WeblateOrg/docker-compose.git
+files:
+  settings-override.py:
+    content: |
+      USE_X_FORWARDED_HOST = True
 override:
   version: "3"
   services:
@@ -18,6 +22,8 @@ override:
         WEBLATE_SITE_DOMAIN: !sat_conf ["", "public_url"]
         WEBLATE_ADMIN_PASSWORD: !sat_generate_pwd
         WEBLATE_ADMIN_EMAIL: !sat_conf ["", "email_admins_list", "", "first"]
+      volumes:
+        - ./settings-override.py:/app/data/settings-override.py:ro
 expose:
   url_prefix: [override, services, weblate, environment, WEBLATE_URL_PREFIX]
   front_url: !sat_param [front_url, /translate]