rolling ubuntu support
As mentioned in #75 (closed), the actually tested Ubuntu docker image is 16.04 (LTS) which is pretty old. This MR provides also latest Ubuntu (regardless LTS status).
- Tested Ubuntu LTS, Ubuntu Latest, Arch latest
- Minor refactor
See https://hub.docker.com/_/ubuntu/
- closes #75 (closed)
Edited by Petr Bělohlávek