I have a nixops network where I deploy containers using the `container`
backend which uses `nixos-container` intenrally to deploy several
containers to a certain host.

During that time I removed and added new containers and while trying to
deploy those to a different host I realized that it isn't guaranteed
that each container gets the same IP address which is a problem as some
parts of the deployment need to know which container is using which IP
(i.e. to configure port forwarding on the host).

With this change you can specify the container's IP like this (and don't
have to use the arbitrarily used 10.233.0.0/16 subnet):

```
$ nixos-container create test --config-file test-container.nix \
    --local-address 10.235.1.2 --host-address 10.235.1.1
```

…tainer

nixos-container: allow setting custom local and host address