chore: migrate CI from Zuul to GitHub Actions

.github/workflows/docker-build.yaml

@@ -0,0 +1,57 @@
+name: Docker Image Build
+
+on:
+  pull_request:
+    types:
+      - opened
+      - reopened
+      - synchronize
+      - closed
+
+env:
+  REGISTRY: quay.io
+  IMAGE: quay.io/stackmon/metrics-processor
+
+jobs:
+  build:
+    if: github.event.pull_request.merged != true
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v7
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Build image
+        uses: docker/build-push-action@v7
+        with:
+          context: .
+          target: metrics-processor
+          push: false
+
+  push_if_merged:
+    # The tag is pinned by stackmon-config/config.yaml, keep change_<pr>_latest.
+    if: github.event.pull_request.merged == true
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v7
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Login to Container Registry
+        uses: docker/login-action@v4
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ secrets.REGISTRY_USER }}
+          password: ${{ secrets.REGISTRY_PASSWORD }}
+
+      - name: Build and push
+        uses: docker/build-push-action@v7
+        with:
+          context: .
+          target: metrics-processor
+          push: true
+          tags: ${{ env.IMAGE }}:change_${{ github.event.pull_request.number }}_latest