Add Dockerfile for Ubuntu 20.04
Related to ghc#18390, to properly test io_uring related code we need an image with a modern enough kernel to have the requisite libraries. Ubuntu 20.04 LTS fits that requirement.
Related to ghc#18390, to properly test io_uring related code we need an image with a modern enough kernel to have the requisite libraries. Ubuntu 20.04 LTS fits that requirement.