fix: correctly assign ffmpeg image tag (#499)

.github/workflows/image.yml

@@ -39,8 +39,8 @@ jobs:
             ffmpeg: ''
           - build-type: ''
             platforms: 'linux/amd64,linux/arm64'
-            tag-latest: 'auto'
-            tag-suffix: ''
+            tag-latest: 'false'
+            tag-suffix: '-ffmpeg'
             ffmpeg: 'true'
           - build-type: 'cublas'
             cuda-major-version: 11