Add Capability for 'Docker Runner' to connect to private image registry

Problem Definition

Currently, Docker Runner only connects to public Docker Image Registries to pull images from, such as hub.docker.com

Please add the capability to connect to private registries to pull images.

Suggested Solution

Allow auth to private registries through input credentials, or the Shared Credentials

Workaround

Add the docker login command to the .profile of the Remote Agent user, for each Agent which will be using the Docker Runner build capability.

docker login -u <username> -p <password> <registryhost:port>

To keep from having passwords stored in plain text in a script you can use the credential helper as outlined in the documentation for docker login