feat: add Docker and Compose support

Add Docker-based deployment: Dockerfile based on systemd image,
docker-compose.yaml, build script, entrypoint, external certificate
monitoring, CI workflow, and documentation.

This builds on the chatmaild/cmdeploy preparation in the previous
commit (j4n/docker-prep-chatmail) which added the env-var-driven
feature flags (CHATMAIL_NOSYSCTL, CHATMAIL_NOPORTCHECK, CHATMAIL_NOACME)
and @local deployment support needed by the container.

This is commit 2 of 3 to merge squashed changes on j4n/docker and docker
branches, original commits were beef0ec..606f36e

Architecture overview (mostly by original author Keonik1):
- Debian-systemd image wrapping the existing cmdeploy install
- Host networking to not manually expose the many ports needed
- Config via MAIL_DOMAIN env var or (new) mounted chatmail.ini
- New: cmdeploy stages: install at build, configure+activate at startup
- New: Monitoring service for external certs via systemd timer (chatmail-certmon)
- New: Image version tracking for automatic upgrade detection (cm + config hash)
- New: docker-compose.override.yaml pattern for user customizations
- New: GitHub Actions CI for ghcr.io image builds

Traefik reverse-proxy support is prepared but the specific files are
excluded from this PR and will be submitted separately.

TODO:
- [ ] Pull out CHATMAIL_NOACME as PR #855 introduced a proper mechanism
- [ ] Check if underlying image could be based on regular debian-slim
  images with a step to enable systemd, similar to
  https://github.com/alexdzyoba/docker-debian-systemd

Files added:
  .dockerignore
  .github/workflows/docker-build.yaml
  docker-compose.yaml
  docker-compose.override.yaml.example
  docker/build.sh
  docker/chatmail_relay.dockerfile
  docker/files/chatmail-certmon.{service,sh,timer}
  docker/files/entrypoint.sh
  docker/files/setup_chatmail.service
  docker/files/setup_chatmail_docker.sh
  env.example
  doc/source/docker.rst

Files modified:
  .gitignore
  doc/source/getting_started.rst
  doc/source/index.rst

Co-authored-by: Keonik1 <keonik.dev@gmail.com>
Co-authored-by: missytake <missytake@systemli.org>

.github/workflows/docker-build.yaml

@@ -0,0 +1,76 @@
+name: Docker Build
+
+on:
+  pull_request:
+    paths:
+      - 'docker/**'
+      - 'docker-compose.yaml'
+      - '.dockerignore'
+      - 'chatmaild/**'
+      - 'cmdeploy/**'
+      - '.github/workflows/docker-build.yaml'
+  push:
+    branches:
+      - main
+      - j4n/docker
+    paths:
+      - 'docker/**'
+      - 'docker-compose.yaml'
+      - '.dockerignore'
+      - 'chatmaild/**'
+      - 'cmdeploy/**'
+      - '.github/workflows/docker-build.yaml'
+    tags:
+      - 'v*'
+
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: ${{ github.repository }}
+
+jobs:
+  build:
+    name: Build Docker image
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Login to GHCR
+        if: github.event_name == 'push'
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract metadata (tags, labels)
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          tags: |
+            # Tagged releases: v1.2.3 → :1.2.3, :1.2, :latest
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            # Branch pushes: j4n/docker → :j4n-docker
+            type=ref,event=branch
+            # Always: :sha-<hash>
+            type=sha
+
+      - name: Build and push
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: docker/chatmail_relay.dockerfile
+          push: ${{ github.event_name == 'push' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+          build-args: |
+            GIT_HASH=${{ github.sha }}