Support DOCKER_AUTH_CONFIG to pull images from custom private registries

GitLab's runner utilizes a DOCKER_AUTH_CONFIG environment variable to access private registries at the job or runner level.

The variable is some escaped JSON that follows Docker's login config format which looks like:

{
    "auths": {
        "registry.example.com:5000": {
            "auth": "bXlfdXNlcm5hbWU6bXlfcGFzc3dvcmQ="
        }
    }
}

…where auths.[reg].auth is base64 encoded basic auth values (i.e., USER:PASS).

So we could match image registries to the keys in the auths object, decode the base64 then split on the colon and supply to cf push.