Ability to specify own pull secret credentials when building images

Currently, Quay and Quay.io do not support adding user pull secrets to builders. This means that pulls of base images can only come from free registries. If the registry is rate limited by the numbner of pulls, such as Docker Hub, this poses a problem because the pull quota could be exhausted quickly if more builds are running concurrently.

By adding the ability to add pull credentials to Quay, we would allow pulls from private registries as well and would alleviate the problem of rate-limiting on Docker Hub (and, possibly, other registries).