Add CI: build+push image to the Gitea container registry on push to main

.gitea/workflows/docker-build-push.yml

@@ -0,0 +1,43 @@
+name: docker-build-push
+
+on:
+  push:
+    branches: [main]
+  workflow_dispatch:
+
+jobs:
+  build-push:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Build and push image via the admin host
+        env:
+          ADMIN_HOST_SSH_KEY: ${{ secrets.ADMIN_HOST_SSH_KEY }}
+          REGISTRY_TOKEN: ${{ secrets.REGISTRY_TOKEN }}
+        run: |
+          mkdir -p ~/.ssh
+          printf '%s\n' "$ADMIN_HOST_SSH_KEY" > ~/.ssh/id_ed25519
+          chmod 600 ~/.ssh/id_ed25519
+          ssh-keyscan -H host.docker.internal >> ~/.ssh/known_hosts 2>/dev/null
+
+          SHA="${{ github.sha }}"
+          REMOTE_DIR="/tmp/ci-yttrx-welcomebot-$SHA"
+          IMAGE="gitea.blairhaus.net/pmb/yttrx-welcomebot"
+
+          tar czf - --exclude=.git . | ssh -i ~/.ssh/id_ed25519 pmb@host.docker.internal \
+            "mkdir -p $REMOTE_DIR && tar xzf - -C $REMOTE_DIR"
+
+          ssh -i ~/.ssh/id_ed25519 pmb@host.docker.internal bash -s <<EOF
+          set -euo pipefail
+          cd "$REMOTE_DIR"
+          echo "==> [host] building $IMAGE:$SHA"
+          docker build -t "$IMAGE:$SHA" -t "$IMAGE:latest" .
+          printf '%s' "$REGISTRY_TOKEN" | docker login gitea.blairhaus.net -u pmb --password-stdin
+          docker push "$IMAGE:$SHA"
+          docker push "$IMAGE:latest"
+          docker logout gitea.blairhaus.net
+          rm -rf "$REMOTE_DIR"
+          EOF
+
+          echo "pushed $IMAGE:$SHA and $IMAGE:latest"