Oleg Grenrus authored Feb 22, 2019 and Ben Gamari committed Feb 22, 2019

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, including
installing GHC in one step (this will prevent sublayers from existing,
making final image slightly smaller)