chore(rust): follow depot recommendations for dockerfiles

[oliverb123]

As per: https://depot.dev/docs/container-builds/how-to-guides/optimal-dockerfiles/rust-dockerfile

First build will be slower since the sccache layer will have to build form scratch, but hopefully from there we'll be quicker

[greptile-apps]

PR Summary

This PR optimizes Rust Dockerfiles by implementing Depot's recommended build caching strategies for improved performance.

• Added sccache as a Rust compiler wrapper with RUSTC_WRAPPER=sccache SCCACHE_DIR=/sccache environment variables
• Implemented Docker build cache mounts with --mount=type=cache for cargo registry, git, and sccache directories
• Switched from lukemathwalker/cargo-chef to rust:1.82 base image with manually installed cargo-chef
• Added --bin $BIN flag to cargo chef commands for more targeted builds
• Maintained the same multi-stage build pattern (planner → builder → runtime) for both main Rust and feature-flags Dockerfiles

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