fix: define and document MAP_BASE_URL to prevent gray maps

[Aletor93]

Checklist

• [x] Any modified or new methods or classes have helpful JSDoc and code is thoroughly commented
• [x] The description lists all applicable issues this PR seeks to resolve
• [x] The description lists any configuration setting(s) that differ from the default settings
• [ ] All tests and CI builds passing

Description

The maps are gray when we start the application with this commit id ae16265b8041d32a45964bb0d208d8eb6cf5a347. A functional fix was suggested by a maintainer. My suggestion is to add it to the default environment variables template.

Fix suggestion: https://github.com/ibi-group/datatools-ui/issues/983#issuecomment-1922182079 Related issue: https://github.com/ibi-group/datatools-ui/issues/983