Added support for private registry authentication

What does this MR do?

Adds support for private registries that require authentication.

Previously, private container image registries had to be available without authentication.

To use this, the user can set:

• CODECLIMATE_REGISTRY_USERNAME
• CODECLIMATE_REGISTRY_PASSWORD
• CODECLIMATE_REGISTRY

Already tested in this project: https://gitlab.com/jdasmarinas/private-registry-codeclimate

• Pipeline that used a Private registry: https://gitlab.com/jdasmarinas/private-registry-codeclimate/-/jobs/2900954630
• Pipeline that used dockerhub (normal behavior): https://gitlab.com/jdasmarinas/private-registry-codeclimate/-/pipelines/616646310

On this example, I used GitLab Dependency Proxy as the "private" registry. After running the job, I verified that it was able to authenticate and pull from GitLab Dependency Proxy:

What are the relevant issue numbers?

• #40 (moved)