Switch over to an alpine based CI image.
Switch over to an alpine based CI image.

I'm trying to make some changes and running afoul of:

    https://github.com/docker/for-linux/issues/480

GHC is easy enough to install on alpine, so rather than fight with the
Debian based Haskell images, I'll just switch to something that doesn't
have this problem.

zenhack/haskell-capnp