libplacebo-ubuntu-focal: add new image

eoan is EOL, focal is the new LTS.

Rather than just bumping all the versions by hand yet again, I decided
to fundamentally rewrite this Dockerfile to use the vulkan sdk focal
repository, which has packages for eveything we need, and should solve
the issue of needing to make sure we pick the correct versions to
prevent stuff from exploding.
1 job for master in 4 minutes and 26 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #518767
build

00:04:26