docker: add HEALTHCHECK, remove VOLUME, fix Dockerfile hygiene

- Added HEALTHCHECK that verifies chatmail services are active via systemctl
- Removed `VOLUME ["/sys/fs/cgroup", "/home"]` as anonymous volumes are
  an anti-pattern for user data (leads to data loss on upgrades). Let
  compose/`docker run -v` handle volume management.
- Changed TZ from Europe/London to UTC (server best practice)
- Removed duplicate WORKDIR /opt/chatmail
- Moved `unlink /etc/nginx/sites-enabled/default` from entrypoint.sh to
  Dockerfile build time

docker/files/entrypoint.sh

@@ -1,8 +1,6 @@
 #!/bin/bash
 set -eo pipefail
 
-unlink /etc/nginx/sites-enabled/default || true
-
 SETUP_CHATMAIL_SERVICE_PATH="${SETUP_CHATMAIL_SERVICE_PATH:-/lib/systemd/system/setup_chatmail.service}"
 
 # Whitelist only the env vars needed by setup_chatmail_docker.sh.