Add CI/CD: multi-arch Docker build workflow

.forgejo/workflows/build.yaml

@@ -0,0 +1,33 @@
+name: Build & Push Docker Image
+
+on:
+  push:
+    branches:
+      - main
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Login to Forgejo Registry
+        uses: docker/login-action@v3
+        with:
+          registry: git.moritz.run
+          username: ${{ secrets.REGISTRY_USER }}
+          password: ${{ secrets.REGISTRY_TOKEN }}
+
+      - name: Build and push
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          push: true
+          platforms: linux/amd64,linux/arm64
+          tags: git.moritz.run/helios/helios-remote:latest
+          cache-from: type=registry,ref=git.moritz.run/helios/helios-remote:cache
+          cache-to: type=registry,ref=git.moritz.run/helios/helios-remote:cache,mode=max