Add flag to preserve all files between builds

[iampava]

Hey,

We're trying to use Gatsby for the build process of a very large and complex website ( > 100k pages), while using our existing Apache server architecture on the back-end. We've seen the Incremental Builds feature, but unfortunately we don't think it helps us.

Ideally we'd be the ones telling Gatsby which pages to rebuild (we already compute this as part of our current setup). We think we can do this via the gatsby-node.js and createPage() API. However, each build removes unused HTML and CSS files from the public folder.

Is there any way to keep them? This way, we could have one full build (initially), and then re-run it on a sub-set of pages.

If there is no such flag right now, could you provide some hints into what to change in the Gatsby code itself? I've looked and found this: https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby/src/services/initialize.ts#L260-L279. Should I "comment" anything else?