CI: Use debian:12-slim with shadow.
This should fix a mismatch between the rust 1.72 docker image and the shadow CI image, which had caused arti to fail to start because it couldn't find libssl3.
Closes #1020 (closed)
This should fix a mismatch between the rust 1.72 docker image and the shadow CI image, which had caused arti to fail to start because it couldn't find libssl3.
Closes #1020 (closed)