Incremental build caching for Rust in builder Docker image

[patchwork01]

Background

Split from:

• Incremental build caching for Rust code in GitHub Actions #2864

Description

We'd like to add incremental build caching for the Rust dependencies, when we run the Rust build in the Sleeper CLI builder image, or in a dev container (using the same Docker image).

This should mean that when we start a new builder from the CLI or in a dev container, we retain the Rust cache from previous invocations.

Analysis

The Sccache support we added in the linked issue sets SCCACHE_DIR, but it uses a local directory inside the container that isn't mounted from the host.

We tried to mount a directory from the host and set that as SCCACHE_DIR, but that didn't seem to work. It may be related to some issues in cross with docker-in-docker:

• Cannot run cross in Docker in Docker in WSL cross-rs/cross#728

We could wait for that to be resolved, or for cross to provide a more reliable way to mount directories into the container. The ideal may be to mount a directory to a fixed path inside the container. Right now it has to be to the same path, but set in an environment variable inside the container.