(maint) Build Docker on Travis Ubuntu 20.04
(maint) Build Docker on Travis Ubuntu 20.04

 - Focal fossa time!
 - Only upgrade the Docker cell as the other cells can't handle focal
   yet
 - No need to upgrade Docker for buildkit as image has 19.03 already,
   so this should remove ~30s of setup time
 - Switch to Ruby 2.6.6 for specs, which is known good and installed
 - docker-compose in the 19.03 release included here is 1.23.1, so
   continue to upgrade it on Travis to 1.25.5

puppetlabs/puppetserver,puppetlabs/puppet-server,puppetlabs/puppetserver,puppetlabs/puppet-server,puppetlabs/puppetserver,puppetlabs/puppet-server,puppetlabs/puppetserver