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>
diff --git a/docker/skia-release/Dockerfile b/docker/skia-release/Dockerfile
index 173fd39..63573c8 100644
--- a/docker/skia-release/Dockerfile
+++ b/docker/skia-release/Dockerfile
@@ -32,7 +32,8 @@
 
 RUN cd /tmp/skia/skia \
   && gclient sync \
-  && ./bin/fetch-gn
+  && ./bin/fetch-gn \
+  && ./bin/fetch-ninja
 
 # Write args.gn.
 RUN mkdir -p /tmp/skia/skia/out/Static
@@ -57,7 +58,7 @@
 RUN cd /tmp/skia/skia \
   && ./bin/gn gen out/Static \
   && git rev-parse HEAD > VERSION \
-  && /tmp/depot_tools/ninja -C out/Static \
+  && ./bin/ninja -C out/Static \
   && chown -R skia:skia . \
   # obj is readable only by the skia user. It needs additional
   # permissions to be accessible for CI (see https://review.skia.org/487217).