[Bug]: Error starting container when using DIND in Kubernetes pod #11139

I updated container-environment detection so we correctly recognize Kubernetes/DIND pods and compute a reachable Docker host IP for unix-socket setups, avoiding “localhost” when inappropriate.
This makes Ryuk and published container ports reachable from your test container in K8s+DIND without requiring manual TESTCONTAINERS_HOST_OVERRIDE.

Key change:
In core/src/main/java/org/testcontainers/dockerclient/DockerClientConfigUtils.java, IN_A_CONTAINER now detects containers using multiple heuristics: /.dockerenv, /run/.containerenv, KUBERNETES_SERVICE_HOST, and cgroup hints (“docker”, “kubepods”, “containerd”, “podman”). When true and DOCKER_HOST is a unix socket, Testcontainers will use the Docker bridge gateway instead of localhost, which fits DIND-in-pod networking.

Author: marktech0813

core/src/main/java/org/testcontainers/dockerclient/DockerClientConfigUtils.java

@@ -6,15 +6,51 @@
 import org.testcontainers.DockerClientFactory;
 
 import java.io.File;
+import java.io.IOException;
 import java.net.URI;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Paths;
 import java.util.Optional;
 import java.util.concurrent.TimeUnit;
 
 @Slf4j
 public class DockerClientConfigUtils {
 
     // See https://github.com/docker/docker/blob/a9fa38b1edf30b23cae3eade0be48b3d4b1de14b/daemon/initlayer/setup_unix.go#L25
-    public static final boolean IN_A_CONTAINER = new File("/.dockerenv").exists();
+	public static final boolean IN_A_CONTAINER = detectInContainer();
+
+	private static boolean detectInContainer() {
+		// Common indicators for being inside a containerized environment:
+		// - Docker: /.dockerenv
+		// - Podman / container tools: /run/.containerenv
+		// - Kubernetes: presence of KUBERNETES_SERVICE_HOST
+		// - cgroup hints: entries containing docker/kubepods/containerd/podman
+		if (new File("/.dockerenv").exists()) {
+			return true;
+		}
+		if (new File("/run/.containerenv").exists()) {
+			return true;
+		}
+		if (System.getenv("KUBERNETES_SERVICE_HOST") != null) {
+			return true;
+		}
+		try {
+			byte[] content = Files.readAllBytes(Paths.get("/proc/1/cgroup"));
+			String cgroup = new String(content, StandardCharsets.UTF_8).toLowerCase();
+			if (
+				cgroup.contains("docker") ||
+				cgroup.contains("kubepods") ||
+				cgroup.contains("containerd") ||
+				cgroup.contains("podman")
+			) {
+				return true;
+			}
+		} catch (IOException ignored) {
+			// ignore and fall back to false
+		}
+		return false;
+	}
 
     @Getter(lazy = true)
     private static final Optional<String> defaultGateway = Optional