Bump base image to Alpine 3.21

[rdwebdesign]

Description

Update Alpine base from 3.20 to 3.21

Note:
The tests/Dockerfile wasn't update yet, because at the moment there is no docker-cli image for alpine3.21 available: https://hub.docker.com/_/docker/tags.

Testing with the previous docker.io/library/docker:27.1.1-cli-alpine3.20 worked.
I also successfully tested with the most recent image (docker.io/library/docker:27.4.0-rc.4-cli-alpine3.20), but we can wait for a -cli-alpine3.21 image.

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