docker: skip redundant cmdeploy run on container restart

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.

docker-compose.yaml

@@ -41,11 +41,11 @@ services:
       ## system (required)
       - /sys/fs/cgroup:/sys/fs/cgroup:rw
       ## data (defaults — override in docker-compose.override.yaml)
-      - chatmail-mail:/home
+      - chatmail-data:/home/vmail
       - chatmail-dkimkeys:/etc/dkimkeys
       - chatmail-acme:/var/lib/acme
 
 volumes:
-  chatmail-mail:
+  chatmail-data:
   chatmail-dkimkeys:
   chatmail-acme: