Option for using self-hosted node:slim

Hello, it's not really a bug, but a feature request.
We experienced this problem due to high amount of pools from docker hub in our IT department:
```
Error response from daemon: toomanyrequests: You have reached your pull rate limit. You may increase the limit by authenticating and upgrading: https://www.docker.com/increase-rate-limit
ERROR.I'm sorry, I couldn't load this CWL file, try again with --debug for more information.
The error was: cwltool requires Node.js engine to evaluate and validate Javascript expressions, but couldn't find it. Tried nodejs, node, docker run node:slim.
```

We are using self-hosted docker images for all our tools but `node:slim`.
Can you add any option for setting which `node:slim` image use for JS validation?