Update Dockerfile to use Focal Fossa to simplify build

[sskeirik]

In Ubuntu Bionic, the provided opam package is too old for building the reference Tezos client. This means we have to use a custom installation method --- but this custom method causes krun to have weird output --- because K's setenv script tries to query opam.

In Ubuntu Focal, the opam package is sufficiently recent. This has two advantages:

• the Dockerfile is simplified because opam installation is not needed
• our CI builds won't have spurious error messages

Since this is a low-priority issue, it makes sense to wait until a K framework Dockerfile based on Ubuntu Focal is shipped.