Reduce image size

As described in #4 (closed)

This doesn't change semantics, but optimizes layers.

It reduced the compressed image size on my local registry from 1388 MB to 894.01 MB.

@bgamari

---

On gitlab registry:

$ docker manifest inspect --insecure registry.gitlab.haskell.org/maerwald/ci-images/x86_64-linux-alpine3_12:26975eeb899bdad04e7df8156d9ce0aea7c9986c | jq '[.la
yers[].size] | add'
1028462892
$ docker manifest inspect --insecure registry.gitlab.haskell.org/ghc/ci-images/x86_64-linux-alpine3_12:4d097a9a8a11adf5576a7434b04f265d0baf8e46 | jq '[.layers[
].size] | add'
1606661362