TestContainers related memory leaks

When running the build I see warnings like this:

:spring-session-sample-javaconfig-redis:jar UP-TO-DATE
:spring-session-sample-javaconfig-redis:integrationTest
:spring-session-sample-javaconfig-redis:appAfterIntegrationTest
19:43:55.657 [localhost-startStop-1] INFO  o.s.w.c.s.AnnotationConfigWebApplicationContext - Closing Root WebApplicationContext: startup date [Thu Sep 07 19:43:50 CDT 2017]; root of context hierarchy
19:43:55.658 [localhost-startStop-1] INFO  o.s.c.s.DefaultLifecycleProcessor - Stopping beans in phase 2147483647

19:43:57 WARN  The web application [ROOT] appears to have started a thread named [dockerjava-netty-1-1] but has failed to stop it. This is very likely to create a memory leak. Stack trace of thread:
 java.lang.Thread.sleep(Native Method)
 org.testcontainers.shaded.io.netty.util.concurrent.SingleThreadEventExecutor.confirmShutdown(SingleThreadEventExecutor.java:705)
 org.testcontainers.shaded.io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:325)
 org.testcontainers.shaded.io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:858)
 org.testcontainers.shaded.io.netty.util.concurrent.DefaultThreadFactory$DefaultRunnableDecorator.run(DefaultThreadFactory.java:138)
 java.lang.Thread.run(Thread.java:745)
19:43:57 WARN  The web application [ROOT] appears to have started a thread named [threadDeathWatcher-3-1] but has failed to stop it. This is very likely to create a memory leak. Stack trace of thread:
 java.lang.Thread.sleep(Native Method)
 org.testcontainers.shaded.io.netty.util.ThreadDeathWatcher$Watcher.run(ThreadDeathWatcher.java:152)
 org.testcontainers.shaded.io.netty.util.concurrent.DefaultThreadFactory$DefaultRunnableDecorator.run(DefaultThreadFactory.java:138)
 java.lang.Thread.run(Thread.java:745)
19:43:57 WARN  The web application [ROOT] appears to have started a thread named [dockerjava-netty-1-2] but has failed to stop it. This is very likely to create a memory leak. Stack trace of thread:
 java.lang.Thread.sleep(Native Method)
 org.testcontainers.shaded.io.netty.util.concurrent.SingleThreadEventExecutor.confirmShutdown(SingleThreadEventExecutor.java:705)
 org.testcontainers.shaded.io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:325)
 org.testcontainers.shaded.io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:858)
 org.testcontainers.shaded.io.netty.util.concurrent.DefaultThreadFactory$DefaultRunnableDecorator.run(DefaultThreadFactory.java:138)
 java.lang.Thread.run(Thread.java:745)

we should ensure all the threads are shutdown properly

Reproduce the warnings while running the build, focusing on the spring-session-sample-javaconfig-redis integrationTest and appAfterIntegrationTest tasks. Use the shown dockerjava-netty and threadDeathWatcher stack traces to trace the TestContainers shutdown lifecycle. Done means the integration test completes without warnings about web-application threads that failed to stop.