CLI: Allow pathPrefix configuration as command line argument

[JamesMGreene]

Describe the feature you'd like to request

When I want to do a production deploy of a static HTML export of a Gatsby site to GitHub Pages, or as a piece of a larger site, I need to:

1. Add a pathPrefix configuration of something like "/repoName/" to the gatsby-config.js file
2. Execute gatsby build with with a flag of --prefix-paths or an environment variable of PREFIX_PATHS=true

I'm seeking a way to more easily automate Gatsby projects across my organization, especially without need to worry about their particular application setup (CommonJS, ESM, TypeScript, etc.). As such, I would like to eliminate the need for the manual editing of the configuration file in Step 1 by combining it into Step 2.

Describe the solution you'd like

I would like to be able to pass the desired pathPrefix configuration value as a command line argument to the necessary gatsby CLI commands, e.g. gatsby build --path-prefix=/repoName/.

Describe alternatives you've considered

The only good alternative I've found is requiring all of the projects to use a consistent snippet to support conditional values, e.g. using an environment variable to modify the pathPrefix value:

module.exports = {
  pathPrefix: process.env.BASE_PATH || '/repoName/'
}

Alas, that requires changes to all of the repositories' codebase as well as participation/enforcement to ensure they remain in that kind of an ideal pattern. 😕

On the plus side, Gatsby is at least closer to what I want than Next.js or NuxtJS, since Gatsby will ignore the pathPrefix unless the flag or environment variable are specified. 🤔

Prior Art

• 11ty's --pathprefix: https://www.11ty.dev/docs/config/#deploy-to-a-subdirectory-with-a-path-prefix
• Jekyll's --baseurl: https://jekyllrb.com/docs/configuration/options/#build-command-options
• Hugo's --baseURL: https://gohugo.io/commands/hugo/
• Proposed feature request for Next.js: CLI: Allow `basePath` configuration as command line argument vercel/next.js#38776
• Proposed feature request for NuxtJS: https://github.com/nuxt/framework/discussions/5976
• etc.