[example] Gatsby sandbox link doesn't work out-of-the-box

[SandersAaronD]

The gatsby example sandbox link doesn't run because sandbox defaults to node 10. It's simple to fork the base codesandbox in order to bump the version with a configuration file, and the readme for the gatsby example should be updated.

• The issue is present in the latest release.
• I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

Build fails while following the codesandbox link in examples/gatsby

Expected Behavior 🤔

Build succeeds when following the codesandbox link

Steps to Reproduce 🕹

Steps:

1. Base codesandbox link goes here: https://codesandbox.io/s/github/mui-org/material-ui/tree/HEAD/examples/gatsby
2. If you follow it, you will get a node version error.

Context 🔦

https://codesandbox.io/s/github/mui-org/material-ui/tree/HEAD/examples/gatsby is my fork that changes nothing except bumping the version in a config file. I can't commit to maintaining an image to keep it up to date, so I haven't put in a PR to redirect the link there.

[eps1lon]

The gatsby examples (examples/gatsby, examples/gatsby-theme), nextjs (examples/nextjs, examples/nextjs-with-typescript) and other SSR examples (examples/ssr need to use node 12 via creating a sandbox.config.json with

{
  "container": {
    "node": "12"
  }
}

The file needs to be next to the corresponding package.json.

[qiweiii]

I would like to take this issue

[oliviertassinari]

A bit related #27331 (comment)