Change default hostname in the docker container

[serbinsh]

When original testing we used "modex" to match the test machine. It is more appropriate to change this hostname to something like "docker" to make it more generic

However, for singularity <3.x the hostname cannot be set at the exec call so custom CIME machines files with specific hostnames (e.g. modex) will need to be used to run on those HPCs

[serbinsh]

New machine files prepared, just need to update the container builds and then alert folks like Andy Fox about the change in hostname

[serbinsh]

Current working location for updating machines files: https://github.com/serbinsh/ctsm_containers/tree/master/cime_config_files/cesm/machines

Will also need to include the mpi serial libs (maybe?) here or just need to update the xmlchange to turn that on during a create_case step

[serbinsh]

its has been decided, the container hostname will be:

clm_container