Add workflow to push image

This is so far some copy&paste work from OpenProject.
Might need further cleanup of duplication.

Author: NobodysNightmare

.github/workflows/release-image.yml

@@ -0,0 +1,43 @@
+name: Release image
+
+on:
+  push:
+    branches:
+      - main
+      - image-release # TODO: testing only
+
+permissions:
+  contents: read # to fetch code (actions/checkout)
+
+env:
+  REGISTRY_IMAGE: openproject/openproject-id
+  KEYCLOAK_VERSION: 26.3.3
+
+jobs:
+  build:
+    if: github.repository == 'opf/openproject-id'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v5
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+      - name: Set up Docker Buildx
+        id: buildx
+        uses: docker/setup-buildx-action@v3
+      - name: Login to Docker Hub
+        uses: docker/login-action@v3
+        with:
+          username: ${{ secrets.DOCKER_USERNAME }}
+          password: ${{ secrets.DOCKER_PASSWORD }}
+      - name: Build & push image
+        id: build
+        uses: docker/build-push-action@v6
+        with:
+          context: image
+          file: image/Containerfile
+          platforms: linux/amd64,linux/arm64
+          build-args: |
+            BUILDKIT_PROGRESS=plain
+          push: true
+          tags: "${{ env.REGISTRY_IMAGE }}:${{ env.KEYCLOAK_VERSION }}"