added AuthConfigs to BuildOptions and pass that to BuildImage

[DGollings]

Please consider this a 'naive PR' - we ran into an issue where although it was possible to pass along authentication to the run commands but not the build.

This fixes it, but I'm wondering why this wasn't already possible or whether I'm doing something else wrong

And i'm wondering why dockertest did not use already present and logged in ~/.docker/config.json

Either way, thanks for dockertest, it has already been used tens of thousands of times by us :)