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/test-and-deploy.yaml

@@ -4,6 +4,7 @@ on:
   push:
     branches:
       - main
+      - j4n/docker-pr
   pull_request:
     paths-ignore:
       - 'scripts/**'
@@ -12,6 +13,11 @@ on:
       - 'LICENSE'
 
 jobs:
+  trigger-docker-build:
+    if: github.event_name == 'push'
+    uses: ./.github/workflows/trigger-docker-build.yaml
+    secrets: inherit
+
   deploy:
     name: deploy on staging2.testrun.org, and run tests
     runs-on: ubuntu-latest
@@ -95,3 +101,12 @@ jobs:
       - name: cmdeploy dns
         run: cmdeploy dns -v
 
+  deploy-docker:
+    needs: [deploy, trigger-docker-build]
+    if: github.event_name == 'push'
+    uses: ./.github/workflows/docker-deploy.yaml
+    with:
+      staging_host: staging2.testrun.org
+      mail_domain: staging2.testrun.org
+      zone_file: staging.testrun.org-default.zone
+    secrets: inherit