Enable adding of pull credentials to builds

With the new changes being brought to Docker Hub, many images are being rate limited because Quay builders are accessing these images as anonymous, non-authorized users. Moving upstream images is sometimes very difficult and other times impossible, especially for smaller users that want to use Quay.io's ability to build images. Specifying pull credentials during build procedure will circumvent Docker's rate limiting and will allow users to continue to build containers on Quay.io and use Quay.io as they did in the past.