Use a separate repo. to store Dockerfiles.

1. Dockerfiles do not depend on the Pegasus repo. itself.
2. Docker builds get triggered on each commit to Pegasus repo., thus every build requires pulling the latest container from Dockerhub.

Dockerfiles have been moved to
    https://github.com/pegasus-isi/pegasus-docker-build
Merge branch 'master' of https://github.com/pegasus-isi/pegasus