Fix builds for cosmic/disco/eoan

Fix builds for cosmic/disco/eoan

Issue:
quassel builds for ubuntu cosmic, disco and eoan fail if running in docker on a xenial host

Reason:
cosmic, disco and eoan use Qt 5.11 and 5.12 respectively, which makes use of the new syscall statx (introduced in Qt 5.10, bionic and xenial are still on 5.9 and 5.5 respectively).
xenial supports statx, as does docker, which is up to date on these distros. but sadly libseccomp, which is used to filter syscalls, is too old on these distros.
so code sees that statx is available, tries to run it, and it returns with an error, which is considered file not existing

Solution:
update the host for our build to at least bionic, where this issue doesn’t exist