Read file from file system in Gatsby Function

[jgeringer]

I am trying to figure out how to read a file from the file system in a Gatsby Function. The code I have now works locally without problem, but not when up on the server. I assume it's because the server only have the built files. To make matters worse, I don't really want to put it in static or anywhere public, as it's a certificate file for server-to-server API calls.

This is my current code that works locally:

import { GatsbyFunctionResponse } from "gatsby"
import fetch from "node-fetch"
import https from "https"
import fs from "fs"
import path from "path"

export default function handler(
    req: null,
    res: GatsbyFunctionResponse
) {
    const pfx = fs.readFileSync(path.resolve(__dirname, `../../cert/myCertFile.p12`));
    const passphrase = `abc123`;
    const agent = new https.Agent({ pfx, passphrase });
    
    fetch(`https://securedapi.com/`, {
        agent
    })
    .then(response => response.json())
    .then(response => {
        res.status(200).json({
            response
        })
    }).catch(error => {
        res.status(500).json({
            error
        })
    })
}

However, I can't seem to get the file when hosted on Netlify at all - have tried all sorts of paths to it.

Any ideas on how I can make it work? I tried to also read the file in as a new node, but then I get error messages about Webpack not being able to parse the file instead.