Dockerfile: Add Ubuntu 22.04 support
Added:
- Ubuntu 22.04 LTS base image, tagged as -VERSION#-ubuntu-22.04 [#71]
Changed:
- Switched Travis environment from the default Ubuntu 16.04 (Xenial) to Ubuntu 20.04 (Focal) and switched arm64 builds from arm64 LXD to arm64-graviton as Travis CI builds were otherwise failing.
- Bump docker-base version from
4.1.0
to5.0.0