RF-19905 Use DockerHub user for pulling down images
RF-19905 Use DockerHub user for pulling down images

Docker are changing their TOS to rate limit unauthenticated `docker pull`
from the start of November. The rate limiting is based on IP, so builds on
Circle will be immediately impacted. Use a paid-for account and contexts to
configure a user to pull down docker images for CI/CD.

rainforestapp/circlemator,rainforestapp/circlemator