MDX tests take way too long

[rnbguy]

MDX tests take too long to set up OCaml and to build ocaml-mdx-test binary. If the caches are not hit, it takes 11 mins on ubuntu and 20 mins on macos.

Does it sound good to publish a pre-compiled docker image on DockerHub and use that in CI?

[shonfeder]

In case this is helpful:

• We've found using github's ghcr to be easier to integrate and maintain than docker hub.
• We are using a nix flake for our integration tests, meaning our cache is very stable. That said, it still takes about 5mins to set up our deps (should be able to accelerate if we use cachix, but this isn't the bottleneck for our integration tests atm).

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Having a quick portable version of mdx seems like it would be helpful!