Refactor Docker login logic to eliminate credential warning.

[MichaelSimons]

WARNING! Your password will be stored unencrypted in /root/.docker/config.json.
Configure a credential helper to remove this warning. See
https://docs.docker.com/engine/reference/commandline/login/#credentials-store

[mthalman]

[Triage] In the build pipeline the container where the credential is used ends up living only temporarily on the build agent is deleted at the end of the process. The credential wouldn't be persisted after that. But for local scenarios that make use of Image Builder this can be considered problematic.