Composability with other actions

[JasonGross]

It might be nice to port fiat-crypto and coq-tools to the docker coq action.

Currently fiat-crypto uses actions/setup-haskell@v1 with ghc 8.8.1 and 3.0 (and uses my ppa for Coq versions 8.12.0, 8.11.1, 8.10.2, 8.9.1, as well as the tips of master, v8.12, v8.11, v8.10, v8.9), and coq-tools uses actions/setup-python@v1 to test python versions 2.7, 3.5, 3.6, 3.7, 3.8 with Coq versions 8.11.1, 8.10.2, 8.9.1, 8.8.2, 8.7.2, 8.6.1, 8.5pl3, and the tips of the master, v8.11, v8.10, v8.9, v8.8, v8.7, v8.6, and v8.5 branches. However, I don't think docker containers are easily composable with other actions.

[JasonGross]

Oops, reported this in the wrong repo, I've re-reported it as coq-community/docker-coq-action#51