Normally, protoc compiler itself is compiled from source code when we use proto_library. However, we can speed up the build by using prebuilt protoc compiler.
Declare a repository for prebuilt protoc compiler in WORKSPACE:
load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
http_archive(
name = "com_google_protobuf_protoc_linux_x86_64",
build_file_content = 'exports_files(["bin/protoc"])',