Add Cargo.lock to git
TLDR: it helps with reproducible builds. https://doc.rust-lang.org/cargo/faq.html#why-have-cargolock-in-version-control
Having it checked in seems like common practise for most binaries. In fact, older versions of the above link definitely advise to put Cargo.lock into git, e.g.: http://web.mit.edu/rust-lang_v1.25/arch/amd64_ubuntu1404/share/doc/rust/html/cargo/faq.html#why-do-binaries-have-cargolock-in-version-control-but-not-libraries