Update Ubuntu Docker container to Ubuntu 20.04

What does this MR do?

Update the Ubuntu version used in the "ubuntu" Docker image

Why was this MR needed?

20.04 is the current LTS version and should be stable enough by now.

What's the best way to test this MR?

Launche the container and check if /etc/os-release contains information for Ubuntu "Focal Fossa".

What are the relevant issue numbers?

None, this is imho a trivial change. I can open an issue if it is really deemed necessary.

Additional notes:

There could be a few more things changed/fixed in this container (e.g. removing apt-transport-https, installing packages like git-lfs and dumb-init from upstream (https://packages.ubuntu.com/focal/git-lfs, https://packages.ubuntu.com/focal/dumb-init), choosing curl OR wget instead of installing both...), if that's something that would be interesting to you, I can open issues/MRs for this.