Support relative base images

[atemate]

We really need to be able to process this:

FROM image:neuromation-base

instead of this:

FROM registry.neuro-compute.org.neu.ro/artemyushkovsky/neuromation-base

Algorithm:
neuro-extras image build -> neuro-extras reads the Dockerfile, if it has FROM image:..., then unwrap this relative name and write it to a temp file. And then build image from this dockerfile.