Add default port for API Swagger docs

[kb2ma]

The swagger-docs target to the top level makefile builds the docs and makes them available on localhost port 9000. Add the port number to the README.

[lmbarros]

The ideal would be to try to get this merged upstream and then get the change via a merge or cherry-pick (to avoid diverging from Moby, and thus simplifying merges). Doesn't look like a controversial thing, so I guess this has a reasonable chance of being accepted.

What do you think, @kb2ma ?

[kb2ma]

I didn't realize this came directly from upstream. Sure, sounds like a worthwhile experiment. Let me first be sure my force-push fixes the error.

[kb2ma]

@lmbarros , the build has failed a couple of times now on the "Is custom" check. Have you seen this error before? Any idea on how to fix it?

[lmbarros]

I don't recall seeing this before, @kb2ma . Our last commit on this repo was almost 2 months ago. Maybe a flowzone regression created over this period?

[kb2ma]

Created moby/moby#46781, so closing this one.