Ability to pull images that Pipelines Runners depend on from private registries

Pipelines Runners builds pull certain public images from DockerHub to be able to run themselves, such as "sox/atlassian/bitbucket-pipelines-dvcs-tools".

This is necessary for Runners to be executed in the first place, regardless if your actual build image is hosted on a private registry. This causes a problem though, as DockerHub will rate-limit users after 200 authenticated requests, effectively causing Pipelines to have a hard cap of 200 builds every 6 hours, unless the user has a paid DockerHub account.

This feature request is to provide the ability for users to fetch the Runners-required images from their private registries, so that they won't get rate-limited by DockerHub.