c++ bindings check capable vm in pull-request build checks

[baallan]

Currently none of the vms used for the pull request checks have g++ installed so that new code dependent on the c++ api can be checked. As a lot of modern c++ code depends on boost libraries, it would be helpful there was at least an ubuntu container with g++ and boost included. It's not obvious to me (perhaps lack of correct access or vm repo url) how to add another preconfigured test vm.