Update gcloud auth command

This is a docs update to update the right command to set up authentication credentials with current gcloud versions.

gcloud auth login does not automatically configure the default credentials, docker-machine create gives this error:

Error with pre-create check: "google: could not find default credentials. See https://cloud.google.com/docs/authentication/external/set-up-adc for more information" 

The right command is gcloud auth application-default login

⚠ ️NOTE THAT THIS FORK IS MAINTAINED FOR CRITICAL BUG FIXES AFFECTING RUNNING COSTS ONLY. NO OTHER CONTRIBUTIONS WILL BE ACCEPTED. ⚠️

What critical bug this MR is fixing?

How does this change help reduce cost of usage? What scale of cost reduction is it?

In what scenarios is this change usable with GitLab Runner's docker+machine executor?