RFC: CI: Use Debian jessie/stretch/buster instead of Ubuntu bionic, cleanups
Generating the docker image hit some spurious failures due to archive.ubuntu.com timing out, see e.g. https://gitlab.freedesktop.org/mesa/mesa/-/jobs/209287 .
Instead of figuring out which Ubuntu mirror should be used instead, I decided to switch to Debian, since that uses a CDN, so should be less prone to this kind of issue.
Pros:
- We can test llvmpipe against LLVM 3.4 vs only down to 3.9 before.
- We get GCC 8 for C and clang 8 for C/C++.
Con:
- We go back from GCC 7 to 6 for C++.
Edited by Michel Dänzer