fix: GATEWAY_URL host address differs between Docker and Podman

[kordless]

Problem

The address a container uses to reach the host depends on the container runtime:

Runtime	Platform	Host address
Docker Desktop	macOS/Windows	host.docker.internal
Docker Engine	Linux	172.17.0.1 (bridge IP) or host.docker.internal if extra-hosts set
Podman rootless	Linux	10.0.2.2 (slirp4netns default)
Podman rootless	macOS	host.containers.internal
Podman rootful	Linux	10.88.0.1 (podman bridge)

When injecting GATEWAY_URL (issue #19), nemesis8 must detect the runtime (already stored in DockerOps::runtime_binary) and platform to choose the right host address.

Fix

Add a gateway_host_address(runtime: &str) -> String helper in docker.rs that returns the correct host address. Use it when building GATEWAY_URL for spawned containers.

For Linux Docker without host.docker.internal, fall back to inspecting the container's default gateway via the Docker API or use --add-host=host.docker.internal:host-gateway in the run args.