Incorrect behavior when running in a Docker container in WSL2

[AaronFriel]

Docker containers do not have access to the Windows environment, so the is-wsl check is not sufficient. The result is that a Docker container on WSL2 fails to run Create React App applications without additional configuration, see logs below.

I was unsure of whether I should make an issue here or in https://github.com/sindresorhus/is-wsl, here's the crux of it:

• Is is-wsl supposed to detect if we're running a WSL1/2 kernel? If so, then it is behaving correctly and open should likely change.

• Or, is is-wsl supposed to detect if we have access from a WSL1/WSL2 userland to Windows utilities, such as cmd.exe? If so, then is-wsl should likely change.

Either way, I have done a bit preliminary research and there are a few reliable ways of detecting if you're running in Docker and WSL2, although different container runtimes can do different things:

1. cat /proc/1/cgroup | grep docker exits with status code 0 if running in Docker, because it runs the init process in various cgroups. (We could also check if it contains more than 1 cgroup, or if it contains cgroups that are not the root cgroup, but I'm less sure of the accuracy of those.)

2. The file /.dockerenv is injected into Docker containers on WSL2 and absent otherwise, but a root user or a distro could theoretically create this file, so it's not reliable.

3. WSL2 distributions do not set the HOSTNAME env var, but Docker on WSL2 does.

4. Lastly, I think this may be the best way to detect if we can access Windows: cat /proc/mounts | awk '{print $3}' | grep drvfs detects whether or not we have a drvfs mount available to us. 🔔 🔔 🔔 this might be the winner.

In which case, I would be happy to make a PR with code like this:

const fs = require('fs');

const parseMountString = (mountString) => {
  const [device, mountPoint, fsType, flags, extra] = mountString.split(' ');

  return { device, mountPoint, fsType, flags, extra };
};

const canAccessWindows = fs
  .readFileSync('/proc/mounts')
  .toString()
  .split('\n')
  .map(parseMountString)
  .some(({ fsType }) => fsType === 'drvfs');

console.log(canAccessWindows);

See result of running this in a Docker container on WSL2 (left) and in a regular WSL2 distribution (right):

Logs showing the error on starting create-react-app application, and a potentially related issue:

> react-app-rewired start

ℹ ｢wds｣: Project is running at http://172.19.0.5/
ℹ ｢wds｣: webpack output is served from /
ℹ ｢wds｣: Content not from webpack is served from /app/public
ℹ ｢wds｣: 404s will fallback to /index.html
Starting the development server...

events.js:174
      throw er; // Unhandled 'error' event
      ^

Error: spawn cmd.exe ENOENT
    at Process.ChildProcess._handle.onexit (internal/child_process.js:240:19)
    at onErrorNT (internal/child_process.js:415:16)
    at process._tickCallback (internal/process/next_tick.js:63:19)
Emitted 'error' event at:
    at Process.ChildProcess._handle.onexit (internal/child_process.js:246:12)
    at onErrorNT (internal/child_process.js:415:16)
    at process._tickCallback (internal/process/next_tick.js:63:19)