Implement HEREDOC functionality from Dockramp

[Tzrlk]

A pretty awesome feature introduced by dockramp was HEREDOC support at the least during execution of RUN statements. While this can be dealt with by mounting a scripts directory, having it in the build file made it much easier to read what was going on.

[ybogdanov]

Thanks for requesting. Unfortunately, we are discontinuing this project. See the notice here: https://github.com/grammarly/rocker/blob/master/README.md

Closing this issue as we are not able to implement it. Sorry.