Use cargo sparse registry in dev container

[hjr3]

For arm64 builds, cargo uses all available memory and often gets killed by the OS. Rust v1.68 has stable support for the sparse registry protocol, which uses much less memory.

Relates to https://github.com/purton-tech/rust-on-nails/issues/14

Additional articles:

• https://blog.rust-lang.org/2023/03/09/Rust-1.68.0.html#cargos-sparse-protocol
• https://yaleman.org/post/2022/2022-10-23-docker-rust-cargo-and-137-errors/
• https://users.rust-lang.org/t/cargo-uses-too-much-memory-being-run-in-qemu/76531