Dynamic Version Generation for Nix Derivations

Replit decided to switch to Nix to solve the Docker image size problem caused by expanding tool and language support. However, Nix has a constraint requiring the version to be specified explicitly in the derivation code, which made it cumbersome to manually update the version every time.

To solve this, they decided to use the short SHA of a Git commit as the version. However, when they tried to run git rev-parse via runCommand, they ran into problems where Nix would execute the command before fetching the source files, or the .git directory was inaccessible due to the isolated environment.

In the end, they tried to solve the problem by using builtins.fetchGit to bring the contents of the local Git repository into the Nix store, and then running git rev-parse within that path to dynamically extract the version.