github actions: build OCI images that contin both amd64 and armv7

- ref. https://docs.docker.com/engine/reference/commandline/buildx_build/#platform
- ref. https://docs.docker.com/build/ci/github-actions/multi-platform/
- replaces https://github.com/shaarli/Shaarli/pull/1496
- make docker image name configurable through CI variables for easier testing

.github/workflows/docker-latest.yml

@@ -35,8 +35,9 @@ jobs:
         with:
           context: .
           push: true
+          platforms: linux/amd64,linux/arm/v7
           tags: |
-            shaarli/shaarli:latest
-            ghcr.io/shaarli/shaarli:latest
+            ${{ secrets.DOCKER_IMAGE }}:latest
+            ghcr.io/${{ secrets.DOCKER_IMAGE }}:latest
       - name: Image digest
         run: echo ${{ steps.docker_build.outputs.digest }}

.github/workflows/docker-tags.yml

@@ -35,8 +35,9 @@ jobs:
         uses: docker/build-push-action@v3
         with:
           push: true
+          platforms: linux/amd64,linux/arm/v7
           tags: |
-            shaarli/shaarli:${{ env.REF }}
-            ghcr.io/shaarli/shaarli:${{ env.REF }}
+            ${{ secrets.DOCKER_IMAGE }}:${{ env.REF }}
+            ghcr.io/${{ secrets.DOCKER_IMAGE }}:${{ env.REF }}
       - name: Image digest
         run: echo ${{ steps.docker_build.outputs.digest }}