`--add-host host.docker.internal:host-gateway` works on Docker Desktop
and standard Docker Engine but not on Rancher Desktop, where the
`host-gateway` keyword is not recognised.
Introduce `_get_host_ip()` which reads the gateway of the Docker bridge
network (e.g. 172.17.0.1) — a portable, concrete IP that works across
Docker Desktop, Docker Engine, and Rancher Desktop. If the network
inspect fails for any reason the function falls back to `host-gateway`,
preserving the existing behaviour for standard installations.
Also add unit tests covering the success path, subprocess failure, and
empty-output fallback.
