Avoid 'Text file busy' race when starting KafkaContainer

Fixes #11682.

The container's command waits for /tmp/testcontainers_start.sh to appear,
then exec's it. containerIsStarting writes the script via a single
Container Archive PUT. On busy hosts the directory entry becomes visible
before the docker engine has released the writer, and the immediate exec
fails with ETXTBSY (exit 126).

Switch to a two-file handshake: write the starter script first, then a
separate /tmp/testcontainers_start.ready marker. The wait loop polls the
marker instead of the script. By the time the second copyFileToContainer
returns, the script's writer is fully closed, so the subsequent exec is
safe. The marker file is never executed, so its own copy can race with
the directory listing harmlessly.

Author: singhvishalkr

modules/kafka/src/main/java/org/testcontainers/kafka/KafkaContainer.java

@@ -72,6 +72,12 @@ protected void containerIsStarting(InspectContainerResponse containerInfo) {
 
         command += "/etc/kafka/docker/run \n";
         copyFileToContainer(Transferable.of(command, 0777), STARTER_SCRIPT);
+        // Marker file: created after the starter script copy has fully released its
+        // writer, so the wait loop in KafkaHelper.COMMAND only proceeds when the
+        // script is safe to execve. Without this, on busy hosts the loop can see the
+        // starter script entry before docker-cp has closed it and fail with
+        // "Text file busy" (exit 126).
+        copyFileToContainer(Transferable.of(""), KafkaHelper.STARTER_SCRIPT_READY_FLAG);
     }
 
     /**

modules/kafka/src/main/java/org/testcontainers/kafka/KafkaHelper.java

@@ -25,10 +25,12 @@ class KafkaHelper {
 
     static final String STARTER_SCRIPT = "/tmp/testcontainers_start.sh";
 
+    static final String STARTER_SCRIPT_READY_FLAG = "/tmp/testcontainers_start.ready";
+
     static final String[] COMMAND = {
         "sh",
         "-c",
-        "while [ ! -f " + STARTER_SCRIPT + " ]; do sleep 0.1; done; " + STARTER_SCRIPT,
+        "while [ ! -f " + STARTER_SCRIPT_READY_FLAG + " ]; do sleep 0.1; done; " + STARTER_SCRIPT,
     };
 
     static final WaitStrategy WAIT_STRATEGY = Wait.forLogMessage(".*Transitioning from RECOVERY to RUNNING.*", 1);