add docker cfngin hook

[ITProKyle]

Summary

Adds a hook to docker image build and docker image push with handling to simplify usage with AWS Elastic Container Registry.

Why This Is Needed

resolves #490

What Changed

Added

• add runway.core.providers.docker to extend the functionality of docker-py
• add runway.lookups.handlers.ecr lookup to get information from either the runway or CFNgin config
  • currently only supports a query of login-password which returns the same value as the awscli command aws ecr get-login-password
• add runway.cfngin.hooks.docker to interact with docker by mimicking the functionality of the docker CLI. The following actions are currently supported
  • docker.login
    • options to simplify ECR authentication
  • docker.image.build
    • options to simplify building an image of ECR
  • docker.image.push
    • options to simplify pushing an image to ECR
• add runway.cfngin.hooks.ecr.purge_repository to remove all images in an ECR repository so it can be deleted by CloudFormation