[algomaster99]

The image from #85 copies Rust from the official image, but COPY recreates /usr/local/cargo and /usr/local/rustup as root-only, so cargo couldn't download crates when the container runs as the host uid. A trial run hit this on cargo test and worked around it with CARGO_HOME=/tmp/cargo-home. This makes both dirs writable again, as the rust image does, and adds clippy and rustfmt, which the rust image leaves out.

before: cargo test
  error: failed to create directory `/usr/local/cargo/registry/cache/index.crates.io-1949cf8c6b5b557f`
  Caused by: Permission denied (os error 13)
before: cargo clippy
  error: 'cargo-clippy' is not installed for the toolchain '1.99.0-x86_64-unknown-linux-gnu'
after: both pass

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