feat(ci): add build docker image gh action

.github/workflows/release-docker.yml

@@ -0,0 +1,34 @@
+name: release/docker
+
+on:
+  push:
+    branches:
+      - main
+
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    steps:
+      -
+        name: Login to Registry
+        uses: docker/login-action@v3
+        with:
+          username: ${{ vars.CONTAINER_REG_USERNAME }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+      -
+        name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+      -
+        name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+      -
+        name: Build and push
+        uses: docker/build-push-action@v6
+        with:
+          platforms:
+            - linux/amd64
+            - linux/arm/v6
+            - linux/arm/v7
+            - linux/arm64/v8
+          push: true
+          tags: ghcr.io/go-vikunja/vikunja:unstable