Add Docker authentication to CircleCI jobs

[bacongobbler]

Starting November 1, 2020, Docker Hub will impose rate limits based on the originating IP. Since CircleCI runs jobs from a shared pool of IPs, it is highly recommended to use authenticated Docker pulls with Docker Hub to avoid rate limit problems.

More information on how to apply authentication to CI jobs can be found here: https://circleci.com/docs/2.0/private-images/

https://github.com/helm/chart-testing/blob/3fc5a5010805e92d099ecacfba57b7c60c12d44a/.circleci/config.yml#L5 https://github.com/helm/chart-testing/blob/3fc5a5010805e92d099ecacfba57b7c60c12d44a/.circleci/config.yml#L18 https://github.com/helm/chart-testing/blob/3fc5a5010805e92d099ecacfba57b7c60c12d44a/.circleci/config.yml#L49

See https://github.com/helm/helm/issues/8848