This is basically a clone of numerous static Nginx buildpacks out there with the added benefit of allowing the project root to be set in an environment variable so your project is not forced to put the root of the static site you are serving in a specific location that may not be ideal for what you are doing.
In your repository create a .env file containing the following:
export BUILDPACK_URL="https://github.com/jamsyoung/dokku-buildpack-nginx-static.git"
Also create a directory named .profile.d. Inside of that directory add a file
named startup.sh. This can actually be named anything, as long as it ends in
.sh, as far as I know. Inside this file, set the location to your
index.html relative to your project root. For most of my projects this is in
public.
export PROJECT_ROOT=public