Dart: Recommend dart build cli over dart compile exe

[simolus3]

This updates the readme for the Dart image to recommend the dart build cli command over dart compile exe. While compile exe only compiles the Dart entrypoint into an executable, dart build cli also respects hooks and is able to bundle dynamic libraries needed by the Dart application or a used library.

I have tested the updated Dockerfile with the template described in Creating a Dart server app, both with and without a package dependency requiring hooks and dynamic libraries.

I've also filed an issue to gather feedback from the Dart team first: dart-lang/dart-docker#253

[dcharkes]

Thank you @simolus3! LGTM!

[athomas]

Thanks, LGTM.

A dart build cli layer may be larger then a dart compile exe layer. Customers that use hooks need to make this change anyway and customers and those that don't will see the same behavior as before.