Cannot use make serve with a custom APP_ENV value

[mrclay]

APP_ENV=ddev make build correctly builds for local API usage, but make serve (technically the serve:watch script) always overwrites the build using APP_ENV=dev.

Expected Behavior

I would expect if I set export APP_ENV=ddev then build and serve to honor that.

Actual Behavior

I have to use make serve.dev to (ironically) serve the DDEV build.

Steps to reproduce the behavior

1. export APP_ENV=ddev
2. make build (ddev config is used)
3. make serve (dev config is used in the watch build)

I guess I need to know how this should work before sending a PR :)