Update 04-builder – Added note regarding use of env and `customPara…

…meters` (#422)

See discussion in game-ci/docker#229

docs/03-github/04-builder.mdx

@@ -227,7 +227,9 @@ Parameters without a value will be considered booleans (with a value of true).
 
 There are 2 main use cases:
 
-- To pass your own custom parameters to be used with `buildMethod` above
+- To pass your own custom parameters to be used with `buildMethod` above. Note that environment
+  variables set in the workflow file will not be available in the environment running Unity, so use
+  this mechanism instead.
 - To pass [Unity Build Options](https://docs.unity3d.com/ScriptReference/BuildOptions.html) (for
   example, `customParameters: -standaloneBuildSubtarget Server` will do server builds)