[Bug]: getFirstMappedPort returns wrong port after adding secondary network

Module

Core

Testcontainers version

1.21.4

Using the latest Testcontainers version?

Yes

Host OS

Linux

Host Arch

x86

Docker version

Version:           29.4.3
 API version:       1.54
 Go version:        go1.26.2
 Git commit:        055a478
 Built:             Wed May  6 17:07:39 2026
 OS/Arch:           linux/amd64
 Context:           default

Server: Docker Engine - Community
 Engine:
  Version:          29.4.3
  API version:      1.54 (minimum version 1.41)
  Go version:       go1.26.2
  Git commit:       56be731
  Built:            Wed May  6 17:07:39 2026
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          v2.2.3
  GitCommit:        77c84241c7cbdd9b4eca2591793e3d4f4317c590
 runc:
  Version:          1.3.5
  GitCommit:        v1.3.5-0-g488fc13e
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

What happened?

I configure a container like so :

Network defaultNetwork = new Network();
Network customNetwork = return new Network() {
				public String getId() { return cystomNetworkId; }
				public void close() {}
				public Statement apply(Statement base, Description description) { return null; }
			};

GenericContainer container = new GenericContainer<>(DockerImageName.parse("registry.ogosecurity.com/repository/ogo/traefik-ogo-proxy:latest"))
					.withNetwork(customNetwork)
					.withNetworkAliases("test-host")
					.withExposedPorts(7077);
container.start();
dockerClient.connectToNetworkCmd()
					.withContainerId(container.getContainerId())
					.withNetworkId(defaultNetwork.getId())
					.exec();

Then, when I ask the mapped port with container.getFirstMappedPort(), the returned port is not usable. docker ps shows a mapped port which is +1 compared to getFirstMappedPort().
It looks like adding a network remaps the exposed port, but testcontainer somewhat cached the first value.
Starting with defaultNetwork and then adding my customNetwork fixes the issue for me regarding the port (I can connect from the host using the given port), but then I can't use the network alias from another container the customNetwork.

Relevant log output

Additional Information

No response

Start with GenericContainer.withNetwork and getFirstMappedPort, then reproduce the supplied setup using connectToNetworkCmd and compare the returned port with docker ps. Trace when the port mapping is obtained relative to adding the secondary network. Done means getFirstMappedPort returns the usable host port after the container is connected to both networks, with a regression test covering the network-alias scenario.