Update CI images to GHC-8.4.4 & cabal-install-2.4.1.0
Use official bindists, except for Debian 9/Stretch http://downloads.haskell.org/debian/ is used. (There are no recent GHC/cabal-install for Debian 8/Jessie there)
Use v2-update/v2-install to install Haskell tools.
Try to unify structure of the different Dockerfiles, incl installing GHC in one step (this will prevent sublayers from existing, making final image slightly smaller)