Rails Premailer Webpacker Using Full Paths

I've ran into this a couple of times and it's taken forever to figure out! I don't know how I fixed it before, but this is how I fixed it this time.

The Problem

When using premailer-rails with webpacker, I run into this error:

ArgumentError: not an HTTP URI

After some digging I found the issue is with the premailer-rails gem. It's trying to request the stylesheet using a full path; file://..... This path is coming from the webpack manifest file. I'm not sure why it's using a full path, but it is.

The Solution

const { webpackConfig, merge } = require('shakapacker')

const customConfig = {
    output: {
        publicPath: process.env["RAILS_ENV"] === "test" ? "/packs-test/" : webpackConfig.output.publicPath,
    }
}

module.exports = merge(webpackConfig, customConfig)

This forces webapck to use a relative path instead of a full path.