Runner to support pulling customer images from a secured private registry

Issue Summary

The Bitbucket Pipelines runner allows users to override the pause, clone and auth proxy images, using an image of your own choosing that implements the same behaviour. 

This may be done for compliance reasons or where modifications to those images need to be made. As an example, your organisation may want to perform security scanning and auditing of those images, push them to a private registry, and only allow the runner to use audited images. 

See https://support.atlassian.com/bitbucket-cloud/docs/use-your-docker-images-in-self-hosted-runners/ for details on how to configure the runner to use your own docker images.

These images can be hosted in a docker registry of your own choosing, however the runner does not support registries that require authentication.

Steps to Reproduce

Follow the steps at https://support.atlassian.com/bitbucket-cloud/docs/use-your-docker-images-in-self-hosted-runners/

Expected Results

We expect to be able to pull images from registries that required authentication.

Actual Results

The runner will fail to execute steps as it is unable to pull images from registries requiring authentication.

Workaround

No work around available.