Show file name and line for each command

[krlmlr]

This helps locating the source of an error, especially if the same command is used several times in the Rockerfile.

[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.