how to build and test with docker

[softprops]

Issue #, if available:

Description of changes:

Expanding the documented options for building a deploy ready lambda artifact. For very simple testing you can use docker directly to both build and invoke your lambda in a local mirror of the AWS provided runtime environment. Like the serverless framework option, this also removes the any need for customizing your developer machine for those that aren't already using serverless framework.

By submitting this pull request

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I confirm that I've made a best effort attempt to update all relevant documentation.

[davidbarsky]

This is excellent, thank you!!!