Too many different images

Currently, we build the whole dune infrastructure with multiple toolchains, e.g. dune-git is built with

• gcc-7-17
• gcc-8[-noassert]-17
• gcc-9-20
• gcc-10-20
• clang-5-17
• clang-6-17
• clang-7-libcpp-17
• clang-10-20

Thus 9 different toolchains. Why? Yes, we should build with different toolchains, like gcc and clang, or c++17 and c++20, but for me it does not make sense to build docker images in all these combinations. Toolchains should be available for all the combinations, yes, but not necessarily docker images.

Dependent images, like dune-pdelab-deps, just use 3 of the configurations.

The images, in my opinions, should not be used as an additional test-suite, but to provide the infrastructure for the other dune modules and their test-suites.

So, I suggest to reduce the images to a minimum:

• dune-git: [gcc-9[-noassert]-17, gcc-10-20, clang-7[-libcpp]-17, clang-10-20]
• dune-2.7: [gcc-7[-noassert]-14, gcc-9-17, clang-7[-libcpp]-17]
• dune-2.6: [gcc-7[-noassert]-14, gcc-9-17, clang-7[-libcpp]-17]
• dune-2.5: [gcc-6-14]
• dune-2.4: [gcc-5-14]

Maybe reduce even more.

This is a reduction by 4 images