Consider using docker-credential-gcr for authenticating to GAR

[dsotirakis]

Source.
auth_token's expiration time is 5 minutes which makes things hard for pipelines that take longer than >5m to finish. What happens is that login-to-gar runs and authenticates successfully, then the next step takes more than 5 minutes to execute and by the time pipeline needs to push to GAR, the token expires and returns 401. This results in users being unable to use our actions.

As an alternative solution, we can use the docker-credential-gcr standalone helper, which can help with authenticating in GCP, for the GAR case (won't work with GCS).

This means, that we'll need to also persist the credentials file until the docker configuration step runs, but we can remove the credential files after the job is done.