(config) Harden Docker deployment for remote servers

Run the Django control panel with Gunicorn instead of the development
runserver and serve static files through WhiteNoise.

Add restart policies, healthchecks, .env-driven production settings, and
a sample .env file for single-server deployments. Update the Docker
entrypoint to collect static assets and document the remote server
deployment and update flow in the README.

.env.example

@@ -0,0 +1,7 @@
+POBSYNC_BACKUP_ROOT=/mnt/backups/pobsync
+POBSYNC_DJANGO_ALLOWED_HOSTS=localhost,127.0.0.1,backup.example.com
+POBSYNC_DJANGO_SECRET_KEY=change-me-to-a-long-random-secret
+POBSYNC_DJANGO_DEBUG=0
+POBSYNC_WEB_BIND=127.0.0.1
+POBSYNC_GUNICORN_WORKERS=2
+POBSYNC_GUNICORN_TIMEOUT=120