Enable use of remote URLs for Swagger files

[markbirbeck]

We started off grabbing the Swagger file from the Docker site and then switched to loading the file locally so as not to be hostage to network connections. But ideally, we'd support both.

[TyIsI]

Not sure if this is still actively being maintained, but I've opened a feature request with the moby repo for this:

moby/moby#46828

I'm happy to open a PR