matrix-sdk 0.17 requires Rust 1.93 (uses Duration::from_mins, declares
rust-version = "1.93"). The container was on 1.90, which is why stories
1022 and 1028 both bounced off the matrix-sdk upgrade despite the host
having Rust 1.93 — the rustup update on the host doesn't propagate into
the build container.
Bumping the FROM rust:1.93-bookworm so the next container rebuild ships
1.93, unblocking matrix-sdk 0.17 upgrades and the rand@0.8 transitive
elimination that comes with it.