[jsfiddle] Run commands in skia-wasm-release container as non-root user.

After this CL lands, //jsfiddle/build will no longer be populated with files owned by the root user.

Change-Id: I289c219b204856e9356e548b70903a19feed0e47
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/417452
Reviewed-by: Leandro Lovisolo <lovisolo@google.com>
Commit-Queue: Leandro Lovisolo <lovisolo@google.com>
diff --git a/jsfiddle/Makefile b/jsfiddle/Makefile
index 0b9b9eb..119b60d 100644
--- a/jsfiddle/Makefile
+++ b/jsfiddle/Makefile
@@ -17,7 +17,9 @@
 	mkdir -p build/canvaskit build/pathkit
 	# Extract them from the most recently built ones. These are built
 	# using cloudbuild.yaml in the Skia repo.
-	docker run --rm --volume `pwd`/build:/OUT gcr.io/skia-public/skia-wasm-release:prod sh -c "cp -r /tmp/* /OUT/"
+	docker run -u $$(id -u ${USER}):$$(id -g ${USER}) --rm \
+	    --volume `pwd`/build:/OUT gcr.io/skia-public/skia-wasm-release:prod \
+	    sh -c "cp -r /tmp/* /OUT/"
 	echo "export const SKIA_VERSION = '`cat build/VERSION`';" > build/version.js
 
 .PHONY: testjs