Add comment to keep Dockerfile up to date against oss-fuzz

tools/dockerfile/test/bazel/Dockerfile

@@ -14,6 +14,12 @@
 
 FROM gcr.io/oss-fuzz-base/base-builder
 
+############################ WARNING ######################################
+# If you are making changes to this file, consider changing
+# https://github.com/google/oss-fuzz/blob/master/projects/grpc/Dockerfile
+# accordingly.
+###########################################################################
+
 # Install basic packages and Bazel dependencies.
 RUN apt-get update && apt-get install -y software-properties-common python-software-properties
 RUN add-apt-repository ppa:webupd8team/java