Include gcloud auth instructions in build-base hint

Change-Id: I351720127fa0c05e077aaa4df0ce472a059701ee

oak_containers/system_image/build-base.sh

@@ -97,3 +97,10 @@ printf "(and/or) bazel run oak_containers/system_image:push_nvidia_base\n\n"
 printf "If you want to use the newly created base, update the hash for\n"
 printf "the oak_containers_sysimage_base oci_pull target (or the nvidia flavour\n"
 printf "if needed) in WORKSPACE\n\n"
+printf "If you haven't done a push before, you'll need to set up gcloud auth:\n"
+printf "Install gcloud if needed: https://cloud.google.com/sdk/docs/downloads-interactive\n"
+printf "And then:\n\n"
+printf "  gcloud auth login\n"
+printf "  gcloud config set project oak-ci\n"
+printf "  gcloud auth configure-docker\n"
+printf "  gcloud auth configure-docker europe-west2-docker.pkg.dev\n"