feat: add CI for docker

Docker and Compose support is provided through a standalone repo at
https://github.com/chatmail/docker, add reusable Docker build/test CI
jobs to staging workflows and a Docker docs stub pointing to the
chatmail/docker repository.

Requires CHATMAIL_DOCKER_DISPATCH_TOKEN secret in relay repo settings
(fine-grained PAT with contents:write on chatmail/docker).

.github/workflows/trigger-docker-build.yaml

@@ -0,0 +1,24 @@
+name: Trigger Docker image build
+
+on:
+  workflow_call:
+
+jobs:
+  trigger-docker-build:
+    name: Trigger Docker image build
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/github-script@v7
+        with:
+          github-token: ${{ secrets.CHATMAIL_DOCKER_DISPATCH_TOKEN }}
+          script: |
+            await github.rest.repos.createDispatchEvent({
+              owner: 'chatmail',
+              repo: 'docker',
+              event_type: 'relay-updated',
+              client_payload: {
+                relay_ref: context.ref,
+                relay_sha: context.sha,
+                relay_sha_short: context.sha.slice(0, 7)
+              }
+            })