add an image for clang-8

[cz4rs]

clang-8 image removed because of failing mpich build:

128.4 checking whether the C++ compiler clang++-8 can build an executable... no
128.4 configure: error: Aborting because C++ compiler does not work.  If you do not need a C++ compiler, configure with --disable-cxx

Originally posted by @cz4rs in #11 (comment)

Add an image with clang-8 for testing vt and magistrate (that's the minimum supported version -> https://github.com/DARMA-tasking/vt/wiki/Build-Requirements#supported-compilers).