Update raker compiler docker image for ruby2.6

kokoro/release/ruby/linux/prepare_build.sh

@@ -9,6 +9,7 @@ sudo service docker restart
 
 # Download Docker images from DockerHub
 export DOCKERHUB_ORGANIZATION=grpctesting
+export RAKE_COMPILER_DOCK_IMAGE=grpctesting/rake-compiler-dock_53c22085d091183c528303791e7771359f699bcf
 
 # All artifacts come here
 mkdir artifacts