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

added 5 commits

• 18ebf39c - 1 commit from branch ghc:master
• 537f85e6 - Don't install GHC from repos on alpine
• 50ff1949 - Optimize layers when installing GHC
• 61d99f3d - Optimize installFromSource for cabal
• 192eeca9 - Run 'cabal update' only once

Compare with previous version

added 4 commits

• 306d424d - Don't install GHC/cabal from repos on alpine
• 0dc0bf13 - Optimize layers when installing GHC
• a79ca077 - Optimize installFromSource for cabal
• 26975eeb - Run 'cabal update' only once

Compare with previous version

changed the description

LGTM. Thanks @maerwald !

approved this merge request

merged

@maerwald, this commit is actually breaking. Specifically, we must cabal update at least twice: once as root to build the bootstrap tools and again as the ghc user (which will be used by downstream GHC builds).