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Have you read the Contributing Guidelines on issues?

• I have read the Contributing Guidelines on issues.

Prerequisites

• I'm using the latest version of Docusaurus.
• I have tried the npm run clear or yarn clear command.
• I have tried rm -rf node_modules yarn.lock package-lock.json and re-installing packages.
• I have tried creating a repro with https://new.docusaurus.io.
• I have read the console error message carefully (if applicable).

Description

Docusaurus is not respecting the $PORT environment variable.

For example if I specify env PORT=8000 yarn start it will start on port 3000 and not on 8000.

Reproducible demo

https://github.com/seyoon20087/docusaurus-reproduction-port-env-not-respecting

Steps to reproduce

1. Clone that repository
2. Install dependencies (yarn)
3. Attempt to start the server at port 8000, using env PORT=8000 yarn start
4. The server will start at port 3000 and not 8000

Expected behavior

I expected that the server should start in the port on the $PORT environment variable specified

Actual behavior

The server will start on 3000 if I don't use --port option.
This is different from other frameworks (like Next.js), which they respect the $PORT environment variable.

For your reference here is the screenshot:

Your environment

• Public source code: https://github.com/seyoon20087/docusaurus-reproduction-port-env-not-respecting
• Public site URL: Not relevant
• Docusaurus version used: 2.3.1 -- note that this issue applies to any current Docusaurus version
• Environment name and version (e.g. Chrome 89, Node.js 16.4): Node.js 16.19.0
• Operating system and version (e.g. Ubuntu 20.04.2 LTS): Gitpod (Ubuntu 20.04.5 container)

Self-service

• I'd be willing to fix this bug myself.