Dockerfile: bump Alpine from 3.18.2 to 3.19.1

[ee7]

Release notes, in chronological order:

• https://alpinelinux.org/posts/Alpine-3.15.10-3.16.7-3.17.5-3.18.3-released.html
• https://alpinelinux.org/posts/Alpine-3.18.4-released.html
• https://alpinelinux.org/posts/Alpine-3.15.11-3.16.8-3.17.6-3.18.5-released.html
• https://alpinelinux.org/posts/Alpine-3.16.9-3.17.7-3.18.6-released.html
• https://alpinelinux.org/posts/Alpine-3.19.0-released.html
• https://alpinelinux.org/posts/Alpine-3.19.1-released.html

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Previous bump: #157

Upstream:

• exercism/nim-docker-base@b71c422
• https://hub.docker.com/layers/exercism/nim-docker-base/b71c422a03821f4764cf5a38d5b1d43f2e4022e5/images/sha256-f0f00a1be7e9de17bfcc5a0e9545b61a62e5b3b60687d8f17b72be41357c11fd