.github, Dockerfile: bump Nim from 2.0.0 to 2.0.2

[ee7]

See the release post and changes.

---

Previous bump: https://github.com/exercism/nim-test-runner/commit/7fba1475e04016f91e5a134e5276e09a0f7cb0ff

Upstream:

• https://github.com/exercism/nim-docker-base/commit/68a971c28c2f5bd44915c1bd24cfd2a5856e5704
• https://hub.docker.com/layers/exercism/nim-docker-base/68a971c28c2f5bd44915c1bd24cfd2a5856e5704/images/sha256-bc998636985028f4d0770f5aaed81cde64ffb429c43e90a36d2e7db54cd259c8?context=explore