Move contributor image creation to GitHub actions

We could speed this up with creating the amd64 image in parallel but the
bottleneck is always the arm64 as GitHub doesn't seem to have arm
runners available yet.

We now create a PR image which can be tested before in case this is
needed.

.github/workflows/build-contributor-container-PR.yml

@@ -0,0 +1,42 @@
+name: 'Build Contributor container (PR)'
+# For speedup to do this in parallel: https://docs.docker.com/build/ci/github-actions/multi-platform/
+# Loosely based on: https://docs.docker.com/build/ci/github-actions/push-multi-registries/
+
+on:
+  push:
+  pull_request_target:
+    branches:
+      - main
+
+jobs:
+  pr-contributor:
+    if: ${{ github.repository != 'domjudge/domjudge-packaging' || github.ref != 'main' }} 
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - 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: ghcr.io
+          username: ${{ github.repository_owner }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract metadata (tags, labels) for Docker
+        id: meta
+        uses: docker/metadata-action@9ec57ed1fcdbf14dcef7dfbe97b2010124a938b7
+        with:
+          images: ghcr.io/${{ github.repository }}
+
+      - name: Build and push
+        uses: docker/build-push-action@v5
+        with:
+          context: docker-contributor
+          platforms: linux/amd64,linux/arm64
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}

.github/workflows/build-contributor-container-release.yml

@@ -0,0 +1,35 @@
+name: 'Build Contributor container (Release)'
+# For speedup to do this in parallel: https://docs.docker.com/build/ci/github-actions/multi-platform/
+# Loosely based on: https://docs.docker.com/build/ci/github-actions/push-multi-registries/
+
+on:
+  push:
+  pull_request_target:
+    branches:
+      - main
+
+jobs:
+  release-contributor:
+    if: ${{ github.repository == 'domjudge/domjudge-packaging' && github.ref == 'main' }} 
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - 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: ghcr.io
+          username: ${{ env.DOCKERHUB_USERNAME }}
+          password: ${{ env.DOCKERHUB_TOKEN }}
+
+      - name: Build and push
+        uses: docker/build-push-action@v5
+        with:
+          context: docker-contributor
+          platforms: linux/amd64,linux/arm64
+          push: true
+          tags: domjudge/domjudge-contributor:latest