Flag to customize gatsby build file name.

[philip-bui]

Hey team,

We wanted to write a flag to allow us to use the exact file name in createPage, instead of /app changing into /app/index.html. We noticed that /app.html works fine into /app.html in the output, but there are certain scenarios where we need it to be the same.

https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby/src/utils/page-html.ts

Background is we are using AWS CloudFront and S3 to host our website, and whenever one of our marketers hit our website (/app?utm_campaign=google), S3 performs a redirect to /app/ (which resolves to /app/index.html), losing our query parameters during that redirect. As there is no S3 solution, we'd like to be able to generate /app HTML files so redirects don't happen.

Wanted to understand what would be required (documentation, testing) etc. before we start on a PR.

Thanks,

[philip-bui]

Here is an existing discussion where they used Lambda at the CloudFront (CDN) layer. We'd like to skip that cost / complexity if possible.

gatsby-uc/gatsby-plugin-s3#51