doc: add port information for using spaces on Docker instance

[valentinRyckaert]

Summary of changes

• Adding info for using Spaces on a Docker instance.

Motivation and context

• When using Docker, Spaces are running on port 3000 like the global TeSS app. This was not specified in the Spaces URL setup doc.

Checklist

• I have read and followed the CONTRIBUTING guide.
• I confirm that I have the authority necessary to make this contribution on behalf of its copyright owner and agree to license it to the TeSS codebase under the BSD license.

[kennethrioja]

I checked with Valentin and on my side before he submitted the PR