First stab at pipeline to build the image

.forgejo/workflows/docker-image.yaml

@@ -0,0 +1,31 @@
+name: docker-image
+
+on:
+  push:
+    branches:
+      - 'main'
+
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    steps:
+      -
+        name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+      -
+        name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+      -
+        name: Login to Docker Hub
+        uses: docker/login-action@v3
+        with:
+          registry: git.hamburg.ccc.de
+          username: ${{ secrets.FORGEJO_REGISTRY_USERNAME }}
+          password: ${{ secrets.FORGEJO_REGISTRY_TOKEN }}
+      -
+        name: Build and push
+        uses: docker/build-push-action@v5
+        with:
+          push: true
+          platforms: linux/amd64,linux/arm64
+          tags: git.hamburg.ccc.de/ccchh/hedgedoc-expire/hedgedoc-expire:latest