message: Create prebuilt bazel-root and bazel-output dirs.
  Bazel is refusing to Mkdir these directories in the Envoy CI builds. I
  don't know how to reproduce it locally, but am hoping it's caused by
  Docker version differences and not some deeper issue.
diff: @@ -26,6 +26,7 @@ echo "Building opt deps ${DEPS}"
  "$(dirname "$0")"/build_and_install_deps.sh ${DEPS}
  echo "Building Bazel-managed deps (//bazel/external:all_external)"
  +mkdir /bazel-prebuilt-root /bazel-prebuilt-output
  BAZEL_OPTIONS="--output_user_root=/bazel-prebuilt-root --output_base=/bazel-prebuilt-output"
  cd /bazel-prebuilt
  for BAZEL_MODE in opt dbg fastbuild; do
  