local dockerized build

CONTRIBUTING.md

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+## development
+
+you'll need a Rust toolchain and various system libraries.
+
+you can obtain the former using `rustup` and the latter using `nix-shell`.
+there's a `shell.nix` provided with the project.
+
+from there, use the commands in the `Justfile`, e.g.:
+
+```sh
+just arranger
+```
+
+note that `tek > 0.2.0-rc.7` will require rust nightly
+for the unstable features `type_alias_impl_trait` and
+`impl_trait_in_assoc_type`. make some noise for lucky
+[**rust rfc2515**](https://github.com/rust-lang/rust/issues/63063)
+if you want to see this buildable with stable/beta.