let GitHub Actions build & publish the Docker image

.github/workflows/build-publish.yml

@@ -0,0 +1,64 @@
+name: build & publish Docker image
+
+on:
+  push:
+    branches:
+      - main
+
+jobs:
+  build-publish:
+    runs-on: ubuntu-latest
+    steps:
+    - name: checkout
+      uses: actions/checkout@v4
+
+    - name: set up Docker buildx
+      uses: docker/setup-buildx-action@v3
+
+    - name: log into the GitHub Container Registry
+      uses: docker/login-action@v3
+      with:
+        registry: ghcr.io
+        username: ${{ github.actor }}
+        password: ${{ github.token }}
+
+    - name: build and push Docker image
+      uses: docker/build-push-action@v5
+      with:
+        load: true
+        tags: ghcr.io/${{ github.repository }}:ci
+        # todo: add linux/arm64 once it is supported by the upstream/base image
+        platforms: linux/amd64
+        # https://docs.docker.com/build/ci/github-actions/cache/#cache-backend-api
+        cache-from: type=gha
+        cache-to: type=gha,mode=max
+
+    - name: compute Docker image tag
+      id: docker-tags
+      # We don't have upstream/base image's tag (or GeoServer version contained, which should be the same) available as a variables, so we must pull it from the image.
+      # todo: find a cleaner solution
+      run: |
+        set -e
+        set -x
+
+        # https://stackoverflow.com/questions/1682442/reading-java-properties-file-from-bash#comment59733051_22857672
+        # https://stackoverflow.com/a/12524345/1072129
+        geoserver_version="$(
+          docker run --rm \
+            'ghcr.io/${{ github.repository }}:ci' \
+            /bin/cat \
+            /usr/local/tomcat/webapps/geoserver/META-INF/maven/org.geoserver.web/gs-web-app/pom.properties \
+            | sed '/^version=/!d; s///' | tr -d '\n'
+        )"
+
+        echo "tag=$geoserver_version" >>$GITHUB_OUTPUT
+
+    - name: build and push Docker image
+      uses: docker/build-push-action@v5
+      with:
+        push: true
+        tags: ghcr.io/${{ github.repository }}:${{ steps.docker-tags.outputs.tag }}
+        platforms: linux/amd64
+        # https://docs.docker.com/build/ci/github-actions/cache/#cache-backend-api
+        cache-from: type=gha
+        cache-to: type=gha,mode=max