Setting up Test Runner (Docker)

To get the [Test Runner](https://exercism.org/docs/building/tooling/test-runners/creating-from-scratch) up we'll need to set up a [Docker instance](https://exercism.org/docs/building/tooling/test-runners/docker).

Currently there are [no plans](https://github.com/dzaima/CBQN/issues/47) to set up official releases with versions for BQN, so we'll likely have to pin our builds to a specific commit. This opens up longer term questions about how we maintain this Docker instance, but I think having anything up and running is probably worth doing first.

There's a reference to a GUIX build [here](https://github.com/mlochbaum/BQN/blob/808f32377aed4fbf62d314d3937a854932e66b58/running.md), but no link. Searching found [references to a build](https://mail.gnu.org/archive/html/guix-patches/2022-08/msg00401.html), but it's not listed [among the packages](https://guix.gnu.org/packages/B/page/2/). Perhaps it hasn't been accepted yet? I don't know much about Guix, but if possible, I'd like to leverage other people's work.

The NIX build I found appears to be [deprecated](https://git.sr.ht/~jshholland/nix-bqn). Not sure if there's another one floating around.

And it's always an option to set up a build from scratch by hook or by crook. Nothing saying we can't change this later.