Each Ubuntu and Debian release has a code name. Ubuntu 18(bionic), Ubuntu 20(focal). This adds the Dockerfile for Ubuntu20.
Next steps and follow up changes:
- add it to the devops site, so we are sure all tests are passing, locally they did
- deploy it to the MCR, naming needs to be clarified, probably just as "focal".
This naming schema allows us in the future to add Debian support too. But we should wait until Headless WK is fixed.
Relates #3791
Relates #2758Closes#3338
