Add clang-8 package to debian-11 Dockerfile to allow toolchain clang-8 to be usable
Summary
In debain-11 image there is a toolchain for clang-8 but no clang-8 compiler installed. This is fixed by this MR.
Discussion
An alternative fix could be to change the toolchain(s) to clang-9, since this is also available in debian-11. I couldn't get both installed at the same time, because of conflicts in the libc++-dev packages.