Locally build your app with the Amplify build image before pushing

**Is your feature request related to a problem? Please describe.**
I'd like to be able to test my amplify.yml build prior to pushing it up.

**Describe the solution you'd like**
A script similar to [AWS CodeBuild's codebuild_build](https://github.com/aws/aws-codebuild-docker-images/tree/master/local_builds)

**Additional context**
Thank you for the build image though. That is helpful.

Start by reviewing the AWS CodeBuild local_builds reference linked in the issue and the Amplify build image described there. Define how a local script should consume amplify.yml and what output should match the hosted build; done means the configuration can be tested locally before pushing.

Written by the indexing model from the issue text.