Add a way to use private repositories on GitHub

[elgohr]

Would be great to have something like this inside the Docker image for working with private repositories.
Especially when running local and there's no other way than vendoring at the moment.

[pr0PM]

@ccojocar I think this would help in resolving the credential issues with gosec GitHub action while run on private repositories too.

tldr;
was just trying to figure out how to make it run on a private repo and noticed these
#490
#732

After googling for a while I noticed this 20h old merge!

[elgohr]

@pr0PM That was also one thought behind it 😁