Dockerfile: prevent unnecessary ffmpeg building (#34084)

.dockerignore

@@ -20,3 +20,9 @@ postgres14
 redis
 elasticsearch
 chart
+.yarn/
+!.yarn/patches
+!.yarn/plugins
+!.yarn/releases
+!.yarn/sdks
+!.yarn/versions

.github/workflows/test-image-build.yml

@@ -8,6 +8,7 @@ on:
       - .github/workflows/test-image-build.yml
       - Dockerfile
       - streaming/Dockerfile
+      - .dockerignore
 permissions:
   contents: read