Currently if you try to clone a repository from github and you don't have any ssh keys configured at github, you will fail to clone and get such message:

Fatal error: fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

Even Permission denied (publickey). message is not shown.

@guyzmo offered to "suggest a

https://github.com/google/starlark-go/blob/master/doc/spec.md

It should satisfy our reproducibility needs (like jsonnet) and could be easier to write agola config files.

Like in #137 we should provide build context information.