Add docker options to repo2docker

Proposed change

Add a way to add docker options to repo2docker.

Alternative options

There are no alternate options AFIK

Who would use this feature?

The issue I am facing is a building a x86 repo2docker on an m1 mac. I think this can be done by using docker options --platform linux/amd64 however I cannot do this currently

How much effort will adding it take?

I think this shoudl be quite straightforward.

Who can do this work?

[yuvipanda]

@manics generally with traitlets I expect them to be loaded from the commandline, we should try to do that

[petersudmant]

Actually, this worked!

jupyter-repo2docker --config traitlets.config.json ./

where traitlets.config.json looks like this

{
    "Repo2Docker": {
        "extra_build_kwargs":{"platform":"linux/amd64"}
    }
}