Ubuntu builds, by default, pull images anonymously from the
official DockerHub. DockerHub rate limits requests from
anonymous and unpaid accounts:
- https://docs.docker.com/docker-hub/download-rate-limit/
When those rate limits are reached the Packer build will
fail with an error indicated the rate limit has been
reached.
Add support for providing credentials for a paid
account that can be used by the Docker setup steps to
authenticate when pulling images from DockerHub, thereby
avoiding the rate limit.
Related to #2094
46e0c05e7e