Add support for lambci/docker-lambda

[jghaines]

node-lambda build gives the following (sensible) warning on macOS:
Warning!!! You are building on a platform that is not 64-bit Linux (darwin.x64). If any of your Node dependencies include C-extensions, they may not work as expected in the Lambda environment.

One way to build on non-Linux platforms is lambci/docker-lambda project that creates a Lambda-like environment in docker.

A node-lambda build --docker option would be great.

[DeviaVir]

#251