Dockerfile fix for 20.04

[g2flyer]

Small fix to dockerfile so they work also with 20.04.
DOCKER_BUILD_OPTS="--build-arg SGX=2.12 --build-arg UBUNTU_VERSION=20.04 --build-arg UBUNTU_NAME=focal" make -C utils/docker
should build all images from utils/docker successfully based on 20.04

See comment on issue #411 for some context.

[mbrandenburger]

Uh this is nice! Though, we may run into similar issues as @bvavala pointed out in #462 that switching between master and flow development may be cumbersome.

[g2flyer]

Though, we may run into similar issues as @bvavala pointed out in #462 that switching between master and flow development may be cumbersome.

This should not affect switching between branches (at least not more than what you have to inevitably do, i.e., rebuild). It also should be backwards compatible if need would arise to backport. In any case, master is effectively frozen before renitegrating flow-refactoring so there are not really any reason to go back and forth? That said, this PR is obviously not urgent, it was just an artifact of stumbling yesterday over the SGX version update and the related apt update break and then multi-tasking during various meetings on assessing the impact of it...

[mbrandenburger]

LGTM! Thanks!