Build cross-platform binaries with Docker

[shinsenter]

This may fix #23.

Tested on various distros.

[tianon]

This doesn't really have anything to do with #23 ? 🤔

Can you elaborate?

[shinsenter]

@tianon
Actually I didn't really modify much in the su-exec.c file.
It was taken from another fork, which claims to have fixed a critical bug from this repo. Please refer to it.
I will update the title of this pull request.

[tianon]

The so-called "critical bug" was fixed upstream in #26 - all that's missing is a release (no further code changes required).

[shinsenter]

@tianon
Sorry about that, I'm not familiar with C.

I've updated the title, and I can revert su-exec.c and Makefile to the original files if you consider the focus of this pull request to be building cross-platform binaries for multiple distros using Docker.

Or just close this pull request, and I'll maintain the binaries and Docker images in my own repository.

Best regards

[tianon]

Sorry, let me be clear: I'm not the maintainer here, just an interested party. 👍

[shinsenter]

Oh, my bad.

I think I'll close this pull request myself, as it doesn't seem to bring much benefit to the su-exec community.

Thank you!