mininet image

[nightlark]

Add a Docker image with mininet installed, probably based on the latest Ubuntu release capable of running HELICS built with all networking cores including MPI, along with OpenSSL and libsodium libraries installed for encryption support. While having compilers for building HELICS could be simplest, rather than adding yet another full build to the CI systems it could be sufficient to have the workflow use a build created in another Docker image that has compilers (possibly from an entirely separate workflow run).