Update FFmpeg setup in CI to build from the workflow directory

Author: Enrico Martelli

.github/workflows/unit-test.yml

@@ -26,7 +26,7 @@ jobs:
       - name: Setup FFmpeg
         shell: bash
         run: |
-          docker run --rm -v "$PWD:/out" -it $(docker build -q .github/workflows/FFmpeg.dockerfile)
+          docker run --rm -v "$PWD:/out" -it $(docker build -q .github/workflows/)
           echo "$PWD" >> "$GITHUB_PATH"
 
       - name: Setup Java