feat(push-to-gar-docker): enable docker mirror for buildx on self-hosted runners

Author: ricky-undeadcoders

actions/push-to-gar-docker/action.yaml

@@ -171,6 +171,7 @@ runs:
       with:
         driver: ${{ inputs.docker-buildx-driver }}
         version: latest # see https://github.com/docker/build-push-action/issues/1345#issuecomment-2770572479
+        buildkitd-config: ${{ runner.environment == 'self-hosted' && '/etc/buildkitd.toml' || '' }}
 
     - name: Build the container
       uses: docker/build-push-action@14487ce63c7a62a4a324b0bfb37086795e31c6c1 # v6.16.0