[CI] Fix the chronik docs build

Summary: The container needs python for the build, which is absent from the node 22 bookworm-slim image. Bump to the larger bookworm to fix it.

Test Plan:

docker build -t chronik-doc -f chronik.e.cash.Dockerfile .

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D18479