Hi,
We have a complex system which produces our own base images from resin’s base images.
We host all our base images ourselves but the resin builders don’t have our registry details.
This means our quite valuable base images have to be made public :S is there any way we can pass the builders credentials? or even better already built images?
Maybe you can give a try to resin local build and deploy. Check out the resin cli, and the resin build and resin deploy commands. These are pretty new and still working on them, hence the lack of big announcement, but if you want to give it a try and give us some feedback, that would be great! 
The resin build command builds your container with your local Docker, and resin deploy sends an image directly to our services without going through the builders. 
now that is some voodoo magic.
Works perfectly.
we have a yarn install inside our dockerfile - am I correct in thinking it’ll retrieve the correct architect from sources? (armv7 while I’m building from my laptop x86)
Awesome.
Yes, by calling resin build with the correct application or device-type/architecture combo (see resin build --help), it should start to build on the right architecture. Then if you have a matching base image in your Dockerfile (the one listed in FROM resin/.....), everything will work together, and can build armv7hf images on x86 just fine. 
Glad to hear it works, let us know if you have any feedback!