Make it possible to build offline

[sr-gi]

It'd be nice to be able to build warnet from a local build, instead of having to pull data from github.

Not sure how viable this is, so feel free to disregard.

[pinheadmz]

Good idea and totally possible, maybe warnet image build can accept either --repo OR --path which would be a local bitcoin repo. I think it would require another (third) dockerfile, and we'd want to trim down the COPY command -- for example I built an image from a local repo recently but it had guix builds and depends crap in the directory and docker was trying to copy literally everything into the image...