Make build dependent on detecting changes on the branch

We only run the workflow when there are changes to the docker folder (or the workflow itself), and we only want to run jobs for platforms that have an updated Docker image to build. Of course we can't detect that change at the job level, so instead we disable the steps to build and push the Docker image when there are no changes for the current platform.

.github/workflows/dockerBuild.yml

@@ -26,11 +26,25 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      - name: Fetch main
+        run: |
+          git remote set-branches --add origin main
+          git fetch --depth 1
+
+      - name: Check for changes
+        id: diff
+        run: |
+          set +e
+          git diff --exit-code --no-patch origin/main docker/nas2d-${{ matrix.platform }}.* ; echo "modified=$?" >> $GITHUB_OUTPUT
+
       - name: Docker build
+        if: ${{ fromJSON(steps.diff.outputs.modified) }}
         run: make -C docker build-image-${{ matrix.platform }}
 
       - name: Docker login
+        if: ${{ fromJSON(steps.diff.outputs.modified) }}
         run: echo "${{ secrets.GITHUB_TOKEN }}" | docker login ghcr.io --username "${{ github.repository_owner }}" --password-stdin
 
       - name: Docker push
+        if: ${{ fromJSON(steps.diff.outputs.modified) }}
         run: make -C docker push-image-${{ matrix.platform }}