Fetch and use ninja when building Skia Release Docker image

ninja is no longer in the depot_tools repo

Change-Id: If4bcc46000c3b751ae813c2e9df63efa0f2e1c72
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/635498
Reviewed-by: Joe Gregorio <jcgregorio@google.com>
1 file changed