Getting the docker container to build and run COSMA tests

[dmikushin]

This set of small changes is required, in order to get the Docker containers to build successfully:

• Use submodule for cxxopts if not available externally, in the same way as it is done for the other dependencies
• Correcting libtree path, which is installed to /opt, not to /root
• Must use libtree v2.0.0, which is the last version where the required --chrpath functionality is available; by some reason, the libtree author decided to remove it in v3.*
• Must provide chrpath package, which the use of libtree depends on

I'm also adding an INSTALL.md note about COSMA installation and testing in Docker.

[dmikushin]

@simonpintarelli kind ping

[dmikushin]

@simonpintarelli , thanks for letting me know. Yes, for cluster-aware module-alike environment Spack is a great choice!

May I please kindly ask you to retain the Docker containers independently of the CI. It is still a worthy option for manual testing and deployment, much easier than local installation.