Fix docker image CI for pull requests (#84)

Closes #65

Pull request images will now be `ttl.sh/techaro/pr-{number}/anubis:24h`.

.github/workflows/docker-pr.yml

@@ -0,0 +1,44 @@
+name: Docker image builds (pull requests)
+
+on:
+  pull_request:
+    branches: [ "main" ]
+
+env:
+  DOCKER_METADATA_SET_OUTPUT_ENV: "true"
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+        with:
+          fetch-tags: true
+          fetch-depth: 0
+
+      - uses: actions/setup-go@v5
+        with:
+          go-version: '1.24.x'
+
+      - uses: ko-build/setup-ko@v0.8
+
+      - name: Docker meta
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ghcr.io/techarohq/anubis
+
+      - name: Build and push
+        id: build
+        run: |
+          go run ./cmd/containerbuild --docker-repo ghcr.io/techarohq/anubis --slog-level debug
+        env:
+          PULL_REQUEST_ID: ${{ github.event.number }}
+
+      - run: |
+          echo "Test this with:"
+          echo "docker pull ${{ steps.build.outputs.docker_image }}"