docs: add auth token guidance for LocalStack module

Starting March 23, 2026, localstack/localstack:latest requires a
LOCALSTACK_AUTH_TOKEN environment variable. Add a warning admonition
and usage examples for both auth token and version pinning approaches.

Author: PreAgile

docs/modules/localstack.md

@@ -2,6 +2,13 @@
 
 Testcontainers module for [LocalStack](http://localstack.cloud/), 'a fully functional local AWS cloud stack', to develop and test your cloud and serverless apps without actually using the cloud.
 
+!!! warning "Auth Token Required for `localstack/localstack:latest`"
+    Starting March 23, 2026, `localstack/localstack:latest` requires a
+    `LOCALSTACK_AUTH_TOKEN` environment variable. To continue without an
+    auth token, pin to a pre-change version such as `4.14.0`.
+    See the [LocalStack Auth Token documentation](https://docs.localstack.cloud/getting-started/auth-token/)
+    for details.
+
 ## Usage example
 
 You can start a LocalStack container instance from any Java application by using:
@@ -10,6 +17,26 @@ You can start a LocalStack container instance from any Java application by using
 [Container creation](../../modules/localstack/src/test/java/org/testcontainers/localstack/LocalStackContainerTest.java) inside_block:container
 <!--/codeinclude-->
 
+### Using an Auth Token
+
+For LocalStack versions that require authentication, set your auth token:
+
+```java
+LocalStackContainer localstack = new LocalStackContainer(
+    DockerImageName.parse("localstack/localstack:latest")
+).withEnv("LOCALSTACK_AUTH_TOKEN", System.getenv("LOCALSTACK_AUTH_TOKEN"));
+```
+
+### Pinning to a Pre-Change Version
+
+To continue without an auth token, pin to a pre-change version such as `4.14.0`:
+
+```java
+LocalStackContainer localstack = new LocalStackContainer(
+    DockerImageName.parse("localstack/localstack:4.14.0")
+);
+```
+
 ## Creating a client using AWS SDK
 
 <!--codeinclude-->