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.yaml

@@ -0,0 +1,48 @@
+# Base compose file — do not edit. Put customizations (data paths, extra
+# volumes, env overrides) in docker-compose.override.yaml instead.
+# See docker-compose.override.yaml.example in this directory for a starting point.
+#
+# Security notes: this container uses 
+#  - network_mode:host chatmail needs many ports (25, 53, 80, 143, 443, 465,
+#  587, 993, 3340, 8443) and needs to operate from the real IP, which bridging
+#  would make tricky
+#  - cgroup:host (required for systemd). 
+#  Together these give the container near-host-level access. This is acceptable
+#  for a dedicated mail server, but be aware that the container can bind any
+#  port and see all host network traffic.
+
+services:
+  chatmail:
+    build:
+      context: ../
+      dockerfile: docker/chatmail_relay.dockerfile
+      args:
+        GIT_HASH: ${GIT_HASH:-unknown}
+    image: chatmail-relay:latest
+    restart: unless-stopped
+    container_name: chatmail
+    # Required for systemd — use only one of the following:
+    cgroup: host          # compose v2
+    # privileged: true    # compose v1 (less restricted)
+    tty: true # required for logs
+    tmpfs: # required for systemd
+      - /tmp
+      - /run
+      - /run/lock
+    logging:
+      driver: none
+    environment:
+      MAIL_DOMAIN: $MAIL_DOMAIN
+    network_mode: "host"
+    volumes:
+      ## system (required)
+      - /sys/fs/cgroup:/sys/fs/cgroup:rw
+      ## data (defaults — override in docker-compose.override.yaml)
+      - mail:/home/vmail
+      - dkim:/etc/dkimkeys
+      - certs:/var/lib/acme
+
+volumes:
+  mail:
+  dkim:
+  certs: