feat: add Docker Compose support

Add container-based deployment as an alternative to bare-metal pyinfra.

- systemd inside container reusing the existing deployer infrastructure
- chatmail-init.sh runs `cmdeploy run --ssh-host @local` on first boot,
  so the container self-deploys using the same code path as bare-metal
- Config via MAIL_DOMAIN env var (simple) or mounted chatmail.ini (advanced)
- External TLS support via TLS_EXTERNAL_CERT_AND_KEY for reverse proxy setups
- Image version tracking in /etc/chatmail-image-version for upgrade detection
- .git/ excluded, but version file mocked so git revparse still works
- Health check verifies postfix, dovecot, and nginx are listening

Files added:
- docker/chatmail_relay.dockerfile: multi-stage build (build + runtime)
- docker/chatmail-init.sh: first-boot deployment script
- docker/chatmail-init.service: systemd unit for init script
- docker/entrypoint.sh: container entrypoint (starts systemd)
- docker/healthcheck.sh: container health check
- docker/docker-compose.yaml: main compose config
- docker/docker-compose.ci.yaml: CI override (uses GHCR image)
- docker/docker-compose.override.yaml.example: customization template
- docker/build.sh: helper script
- doc/source/docker.rst: documentation
- .dockerignore: build context filter

docker/docker-compose.override.yaml.example

@@ -0,0 +1,44 @@
+# Local overrides: copy to docker-compose.override.yaml in this directory.
+# Compose automatically merges this with docker-compose.yaml.
+#
+#   cp docker-compose.override.yaml.example docker-compose.override.yaml
+#
+# Volumes are APPENDED to the base file's volumes list, environment and other scalar keys are MERGED by key.
+services:
+  chatmail:
+    volumes:
+      ## Data paths — bind-mount to host directories for easy access/backup.
+
+      # - ./data/dkim:/etc/dkimkeys
+      # - ./data/certs:/var/lib/acme
+
+      # - ./data/mail:/home/vmail
+      ## Or mount from an existing bare-metal install.
+      # - /home/vmail:/home/vmail
+
+      ## Mount your own chatmail.ini (skips auto-generation):
+      # - ./chatmail.ini:/etc/chatmail/chatmail.ini
+
+      ## Custom website:
+      # - ./custom/www:/opt/chatmail-www
+
+      ## Debug — mount scripts for live editing:
+      # - ./chatmail-init.sh:/chatmail-init.sh
+      # - ./entrypoint.sh:/entrypoint.sh
+
+    # environment:
+      ## Mount certs (above) and set TLS_EXTERNAL_CERT_AND_KEY to in-container paths.
+      ## A tls-cert-reload.path watcher inside the container reloads services
+      ## when the cert file changes. However, inotify does not cross bind-mount
+      ## boundaries, so host-side renewals (certbot, acmetool, etc.) must
+      ## notify the container explicitly. Add this to your renewal hook:
+      ##
+      ##   docker exec chatmail systemctl start tls-cert-reload.service
+      ##
+      ## Host acmetool (bare-metal migration): create mount above, and
+      ##   rsync -a /var/lib/acme/live data/certs
+      # TLS_EXTERNAL_CERT_AND_KEY: "/var/lib/acme/live/${MAIL_DOMAIN}/fullchain /var/lib/acme/live/${MAIL_DOMAIN}/privkey"
+      ##
+      ## (Untested) Traefik certs-dumper (see docker/docker-compose-traefik.yaml) - also add volume:
+      ##   - traefik-certs:/certs:ro
+      # TLS_EXTERNAL_CERT_AND_KEY: "/certs/${MAIL_DOMAIN}/certificate.crt /certs/${MAIL_DOMAIN}/privatekey.key"