fix(docker): bump builder to rust:1.90 to satisfy dep MSRV

[AndreaDiazCorreia]

Summary

• Docker builder was pinned to rust:1.83, but recent transitive deps require newer rustc: getrandom 0.4.2 needs edition2024 (Cargo 1.85+) and time 0.3.47 requires rustc 1.88+. Production deploys were failing on cargo build --release inside the image.
• Bumps the builder to rust:1.90, which clears both MSRV bumps with margin and stays below CI's stable toolchain.
• Updates CLAUDE.md tech-stack line to reflect the new pin.

Test plan

• ./deploy-fly.sh completes the Depot build and the app starts on Fly
• curl https://mostro-push-server.fly.dev/api/health returns OK
• curl https://mostro-push-server.fly.dev/api/info returns the expected payload

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📥 Commits

Reviewing files that changed from the base of the PR and between efc357d and 7dfbbe0.

📒 Files selected for processing (2)

• CLAUDE.md
• Dockerfile

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/docker-builder-rust-1.85

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Keep builder and runtime on same Debian release

On the Fly deploy path I checked (fly.toml builds this Dockerfile), rust:1.90 now resolves to the Trixie-based Rust image while the final stage remains debian:bookworm-slim. Because the release binary is dynamically linked against the builder's libc/OpenSSL stack and then copied into an older Bookworm runtime, deploys can build successfully but fail at container start with missing/newer symbol versions. Pin the builder to the matching suite, e.g. rust:1.90-bookworm, or move the runtime stage to Trixie as well.

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