Patched Caddy NixOS flake

Caddy's third-party plugins are installed by adding them as import in cmd/caddy/main.go and compiling caddy. This can be done either using the xcaddy utility as described here in the official docs or by creating a main.go file with the import and compiling with go build manually. This process is outlined in the upstream here. The xcaddy utility is not suited for deployment on NixOS where a sandboxed, reproducible build is required.

This flake compiles caddy from a custom main.go file as outlined above. Currently adding the popular caddy-security as an example. The caddy package of this flake's output will be caddy with that plugin baked in.

To modify/add plugins:

1. Edit caddy-src/main.go as per the upstream docs
2. Run go mod tidy
3. If necessary, update the hash in flake.nix
4. Run nix build

You should get a result with the compiled caddy. To verify that the plugins where correctly added use:

./result/bin/caddy list-modules