Containerize CI builds

[ulysses4ever]

For this patch I

• add a Dockerfile,
• build an image from the Dockerfile locally and push it under my account on github
• switch CI scripts to use the image.

The only weak point is the second one: if we ever need to update the image, it will require my participation. Or someone else can host it and switch CI scripts again (which should be easy).

CI timings are down to:

• linear builds: 1 min
• liquid: 13 mins

It used to be 14 and 23.

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For posterity, here's how to build and push the image:

❯ cd lh-array-sort-new
❯ docker build -t ghcr.io/ulysses4ever/lh-array-sort .
❯ docker push ghcr.io/ulysses4ever/lh-array-sort:latest

The image needs to be rebuilt whenever we change the dependency tree somehow. This includes updates to the version of GHC we're using.