Add multisite support to the local Docker environment

[desrosj]

The local Docker development environment should support multisite.

Since custom port numbers are not currently supported by multisite installs, this is blocked by #21077.

[johnbillion]

Reviewing as part of the PRs on #21077.

[johnbillion]

In 58097:

Bootstrap/Load: Add support for custom ports in multisite site addresses.

This allows a Multisite network to use an address that includes a port name, such as example.com:1234, and adds support for this to the local development environment too. You can now run a Multisite installation on the local development environment, for example at localhost:8889.

This also fixes some bugs with running a single site installation on a port, and updates the testing infrastructure so that the whole test suite runs both with and without a port number.

Props djzone, scribu, nacin, ipstenu, F J Kaiser, jeremyfelt, johnjamesjacoby, spacedmonkey, PerS, Clorith, Blackbam, enrico.sorcinelli, Jules Colle, obliviousharmony, desrosj, johnbillion

Fixes #21077, #52088