Update FFmpeg setup in CI to use dedicated Dockerfile

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 $(docker build -q .github/workflows/)
+          docker run --rm -v "$PWD:/out" -it $(docker build -q -f .github/workflows/FFmpeg.dockerfile)
           echo "$PWD" >> "$GITHUB_PATH"
 
       - name: Setup Java