Update Dockerfile for optimisation and old clang

[Willenst]

Hello, I've decided that Dockerfile can be optimized by removing duplicate gcc and g++ versions. Since they are just meta packages containing the one specified version of distinct package. You can check it by searching gcc in this repo http://archive.ubuntu.com/ubuntu/dists/focal/main/binary-amd64/

The economy of space will be really nice:

	old	new
kernel-build-container clang-17	6.02GB	4.89GB
kernel-build-container clang-15	3.83GB	3.49GB
kernel-build-container clang-14	2.21GB	1.88GB
kernel-build-container clang-10	1.38GB	1.38GB
kernel-build-container clang-8	1.75GB	1.4GB
kernel-build-container clang-7	2.04GB	1.49GB

Also, I've added support for old clangs, so that they can be added in future! To be honest, I will try to add them really soon!

[a13xp0p0v]

Excellent finding, thanks @Willenst.
I've added a fix be11369 and merged this branch.