Replace the old IMAGE_VERSION_FILE/RUNNING_VERSION_FILE mechanism with a
single deploy fingerprint (image_version:sha256(chatmail.ini)) stored at
/etc/chatmail/.deploy-fingerprint. On restart, if the fingerprint matches
the last successful deploy, skip cmdeploy run entirely. The fingerprint
lives on the container's writable layer. On fresh containers, setting
CMDEPLOY_STAGES non-empty in env forces a deploy run regardless of
fingerprint.
Also narrow the /home volume mount to /home/vmail.
The base docker-compose.yaml was checked into git and thus would get
overwritten on pull.
- docker-compose.yaml uses named volumes as safe defaults
- docker-compose.override.yaml (gitignored) holds user customizations
- Compose automatically merges both files