Debug environment variables when setting up GatsbyJS with headless Wordpress
1. Add to `gatsby-config.js`:
```
require('dotenv').config({
path: `.env.${process.env.NODE_ENV}`
});
```
https://github.com/gatsbyjs/gatsby/issues/3896#issuecomment-364058307
Most common values are development or production, so match them in the env filename
2. Create the env file: .env.development and/or .env.production
https://stackoverflow.com/questions/53741674/cant-access-gatsby-environment-variables-on-the-client-side/53745249#53745249. Use the format described in https://www.gatsbyjs.com/docs/how-to/local-development/environment-variables/
```
WPGRAPHQL_URL=https://example.com/graphql
```
3. Debug using `console.log()`:
```
console.log({
'process.env.NODE_ENV': process.env.NODE_ENV,
'process.env.WPGRAPHQL_URL': process.env.WPGRAPHQL_URL
});
```