Output environment variables during build

[iloveitaly]

Feature request

Adding a section to this display:

Motivation

Inform developers about the default set of environment variables in the build. This would make it easy for them to understand which variables might be worth overriding to a different value, as well as easily introspecting what dynamic defaults were chosen for particular variables.

Right now the only way to investigate this is to Docker inspect the resulting image and look through the massive JSON output.

Related to: #1063

Contribution

I can add this.