Add Alpine 3.23 and 3.24

[f3l1x]

Adds base images for Alpine 3.23 and 3.24. Both are published upstream (latest patches 3.23.5 and 3.24.1), so this brings the repo up to current.

Changes

• 3.23/Dockerfile and 3.24/Dockerfile — byte-identical to 3.22/Dockerfile apart from the FROM line, so the dfx / UID 1000 user, the apk update && upgrade, and the existing ENV TERM xterm style all carry over unchanged. Deliberately copied from 3.22, not from edge, since edge/Dockerfile still has a deprecated MAINTAINER line.
• Two new entries in the workflow build matrix.
• Two new rows in the README table.

Verified that every matrix entry has a matching Dockerfile and that no version directory is missing from the matrix (3.1–3.6 remain commented out, as before).

Why now

dockette/ci's node24 image builds FROM dockette/alpine:3.23, which does not exist on Docker Hub yet — the newest published tag is 3.22. That is currently breaking CI in dockette/ci (see dockette/ci#17). Once this merges and master publishes dockette/alpine:3.23, that failure resolves with no change needed in dockette/ci.

Notes

• Unverified by CI. This workflow is on: push: branches: [master], so nothing builds on a branch — 3.23 and 3.24 are first exercised when this merges. If you want branch builds without publishing, the same change applied in Add PCOV extension to all PHP Docker images ci#17 and Add pcov to PHP 7.1-8.4 images php#27 works here: drop the branch filter and gate the push step on github.ref == 'refs/heads/master'.
• The README's Image column reads dockette/alpine:v3.22 while the workflow actually tags images as 3.22, with no v prefix. That predates this PR and applies to every row; the new rows follow the existing format for consistency rather than introducing a second convention. Happy to correct the whole column separately.

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