build: add Electron linux Dockerfile + auto build image

[firstred]

Adds an Electron builder Docker file for Linux images.

This Docker file includes downloading nvm and installing node 12 LTS so that all environments are running the same version.

[Crecket]

Not sure if we should handle that dockerfile ourselves, I'd rather leave that kind of stuff to the electron devs if possible

[firstred]

Yeah, you're making a good point. I have been wondering myself if this was the correct approach.

We could also use that Dockerfile to pull their image (electronuserland/builder:wine) and only swap the node version so that they are still in control of the whole image, and we can always decide which node version to use.

[firstred]

Is this better?