Dynamic env vars

Was just going through documentation on this page https://db-migrate.readthedocs.io/en/latest/Getting%20Started/configuration/

and trying to create dynamic configruations but the documentation is not very clear, I can help in changing that but could you please shade some light on this.

Im basically trying to have something like
```

{
"defaultEnv": "local",
"local": {
"driver": "pg",
"user": "local_user",
"password": "cor_dev_pass",
"host": "localhost",
"database": "cor_local",
"port": "5432"

},
"development": {
"driver": "pg",
"user": "root",
"password": "root"
},
"staging": {
"driver": "pg",
"user": "root",
"password": "root"
},
"prod": {
"driver": "pg",
"user": "root",
"password": "root"
},

"sql-file" : true

}

```

But documentation does not really explain how will db-migrate KNOW when its local,dev,stag or prod

whats the work around it?

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