tools: github actions: build docker images on pull requests

- fixes https://github.com/shaarli/Shaarli/issues/1800
- do not push, only check that the image builds correctly
- tag the image as :pr-PR_NUMBER

.github/workflows/docker-pr.yml

@@ -0,0 +1,21 @@
+name: Build Docker image (Pull Request)
+on:
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  docker-build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v1
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v1
+      - name: Build Docker image
+        id: docker_build
+        uses: docker/build-push-action@v2
+        with:
+          push: false
+          tags: shaarli/shaarli:pr-${{ github.event.number }}
+      - name: Image digest
+        run: echo ${{ steps.docker_build.outputs.digest }}