Support way to opt into beta image

[agjohnson]

Instead of asking users to sign up for access to get our beta build image, perhaps there is something we can add to the yaml configuration that swaps to the latest build image.

I have concerns about giving users access to the build image name, and I don't want to support more than 2 build images for simplicity, so I think a binary option makes sense here. If there is a strong reason to allow access to all three images, perhaps this could simply be the docker image tag -- ie: 1.0, 2.0, or latest

A couple options:

build_image: 1.0
build_image: 2.0
build_iamge: latest

or

build_image: beta
build_image: standard

or, considering there might be some additional beta features we want to enable on builds:

beta: True
beta: False