Dockerfile: bump Alpine from 3.17.0 to 3.18.2

[ee7]

Release notes:

• https://alpinelinux.org/posts/Alpine-3.17.1-released.html
• https://alpinelinux.org/posts/Alpine-3.17.2-released.html
• https://alpinelinux.org/posts/Alpine-3.17.3-released.html
• https://alpinelinux.org/posts/Alpine-3.18.0-released.htm
• https://alpinelinux.org/posts/Alpine-3.15.9-3.16.6-3.17.4-3.18.2-released.html

(It looks like there was no proper 3.18.1 release).

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Previous bump: https://github.com/exercism/nim-test-runner/pull/142

Upstream:

• https://github.com/exercism/nim-docker-base/commit/baf2cd14c442dab78890019461463ae5a1c64d6c
• https://hub.docker.com/layers/exercism/nim-docker-base/baf2cd14c442dab78890019461463ae5a1c64d6c/images/sha256-472e0019d6e629afb2e89a498672a07ada416fc9ecc65581924dbe594ed43e3c