cluster mode for gatsby serve - Something is already running at port 9000

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Preliminary Checks

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Description

I want to stably serve gatsby applications using tools like pm2, but I can't because gatsby throws an error when trying to run subsequent instances on the same port, probably due to some internal limitations.

Reproduction Link

https://githubbox.com/gatsbyjs/gatsby-starter-minimal

Steps to Reproduce

1. Creating a new PM2 app with 'cluster' mode for the 'gatsby serve' command.
2. run 'pm2 reload gatsby-app'.

Expected Result

reload aplication in cluster mode works without an issue

Actual Result

Gatsby throws "Something is already running at port ..." error.

Environment

Not connected to envrioment.

Config Flags

No response