Cannot install plugin ratelimit

Here is the command used:

```
docker build --build-arg plugins=ratelimit -t local/caddy github.com/abiosoft/caddy-docker.git
```

and the ouput error:

> can't load package: package caddy:
> ratelimit.go:2:10: invalid import path: ""
> error at 'building caddy'

The plugin name `ratelimit` is found when I test with `caddyplug install ratelimit`.

Any idea what the problem is? how can I build this great caddy image with the ratelimit plugin?