Memory leaks reported when using TestContainers as Spring beans.

This was originally reported as spring-projects/spring-session#869.

We're using TestContainers as Spring beans in some integration testing scenarios, as explained in #369. The support for this use case was improved with the latest 1.7.1 release, however we're still experiencing some memory leak log warnings:

Apr 20, 2018 2:26:54 PM org.apache.catalina.loader.WebappClassLoaderBase clearReferencesThreads
WARNING: The web application [ROOT] appears to have started a thread named [ducttape-0] but has failed to stop it. This is very likely to create a memory leak. Stack trace of thread:
 sun.misc.Unsafe.park(Native Method)
 java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:215)
 java.util.concurrent.SynchronousQueue$TransferStack.awaitFulfill(SynchronousQueue.java:460)
 java.util.concurrent.SynchronousQueue$TransferStack.transfer(SynchronousQueue.java:362)
 java.util.concurrent.SynchronousQueue.poll(SynchronousQueue.java:941)
 java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1073)
 java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1134)
 java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
 java.lang.Thread.run(Thread.java:748)
Apr 20, 2018 2:26:54 PM org.apache.catalina.loader.WebappClassLoaderBase clearReferencesThreads
WARNING: The web application [ROOT] appears to have started a thread named [testcontainers-netty-1-1] but has failed to stop it. This is very likely to create a memory leak. Stack trace of thread:
 org.testcontainers.shaded.io.netty.channel.epoll.Native.epollWait0(Native Method)
 org.testcontainers.shaded.io.netty.channel.epoll.Native.epollWait(Native.java:111)
 org.testcontainers.shaded.io.netty.channel.epoll.EpollEventLoop.epollWait(EpollEventLoop.java:231)
 org.testcontainers.shaded.io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:255)
 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:748)
Apr 20, 2018 2:26:54 PM org.apache.catalina.loader.WebappClassLoaderBase clearReferencesThreads
WARNING: 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:748)
Apr 20, 2018 2:26:54 PM org.apache.catalina.loader.WebappClassLoaderBase clearReferencesThreads
WARNING: The web application [ROOT] appears to have started a thread named [testcontainers-netty-1-2] but has failed to stop it. This is very likely to create a memory leak. Stack trace of thread:
 org.testcontainers.shaded.io.netty.channel.epoll.Native.epollWait0(Native Method)
 org.testcontainers.shaded.io.netty.channel.epoll.Native.epollWait(Native.java:111)
 org.testcontainers.shaded.io.netty.channel.epoll.EpollEventLoop.epollWait(EpollEventLoop.java:231)
 org.testcontainers.shaded.io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:255)
 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:748)
Apr 20, 2018 2:26:54 PM org.apache.catalina.loader.WebappClassLoaderBase clearReferencesThreads
WARNING: The web application [ROOT] appears to have started a thread named [testcontainers-netty-1-3] but has failed to stop it. This is very likely to create a memory leak. Stack trace of thread:
 org.testcontainers.shaded.io.netty.channel.epoll.Native.epollWait0(Native Method)
 org.testcontainers.shaded.io.netty.channel.epoll.Native.epollWait(Native.java:111)
 org.testcontainers.shaded.io.netty.channel.epoll.EpollEventLoop.epollWait(EpollEventLoop.java:231)
 org.testcontainers.shaded.io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:255)
 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:748)
Apr 20, 2018 2:26:54 PM org.apache.catalina.loader.WebappClassLoaderBase clearReferencesThreads
WARNING: The web application [ROOT] appears to have started a thread named [testcontainers-netty-1-4] but has failed to stop it. This is very likely to create a memory leak. Stack trace of thread:
 org.testcontainers.shaded.io.netty.channel.epoll.Native.epollWait0(Native Method)
 org.testcontainers.shaded.io.netty.channel.epoll.Native.epollWait(Native.java:111)
 org.testcontainers.shaded.io.netty.channel.epoll.EpollEventLoop.epollWait(EpollEventLoop.java:231)
 org.testcontainers.shaded.io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:255)
 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:748)
Apr 20, 2018 2:26:54 PM org.apache.catalina.loader.WebappClassLoaderBase clearReferencesThreads
WARNING: The web application [ROOT] appears to have started a thread named [testcontainers-netty-1-5] but has failed to stop it. This is very likely to create a memory leak. Stack trace of thread:
 org.testcontainers.shaded.io.netty.channel.epoll.Native.epollWait0(Native Method)
 org.testcontainers.shaded.io.netty.channel.epoll.Native.epollWait(Native.java:111)
 org.testcontainers.shaded.io.netty.channel.epoll.EpollEventLoop.epollWait(EpollEventLoop.java:231)
 org.testcontainers.shaded.io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:255)
 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:748)
Apr 20, 2018 2:26:54 PM org.apache.catalina.loader.WebappClassLoaderBase clearReferencesThreads
WARNING: The web application [ROOT] appears to have started a thread named [testcontainers-netty-1-6] but has failed to stop it. This is very likely to create a memory leak. Stack trace of thread:
 org.testcontainers.shaded.io.netty.channel.epoll.Native.epollWait0(Native Method)
 org.testcontainers.shaded.io.netty.channel.epoll.Native.epollWait(Native.java:111)
 org.testcontainers.shaded.io.netty.channel.epoll.EpollEventLoop.epollWait(EpollEventLoop.java:231)
 org.testcontainers.shaded.io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:255)
 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:748)
Apr 20, 2018 2:26:54 PM org.apache.catalina.loader.WebappClassLoaderBase clearReferencesThreads
WARNING: The web application [ROOT] appears to have started a thread named [testcontainers-netty-1-7] but has failed to stop it. This is very likely to create a memory leak. Stack trace of thread:
 org.testcontainers.shaded.io.netty.channel.epoll.Native.epollWait0(Native Method)
 org.testcontainers.shaded.io.netty.channel.epoll.Native.epollWait(Native.java:111)
 org.testcontainers.shaded.io.netty.channel.epoll.EpollEventLoop.epollWait(EpollEventLoop.java:231)
 org.testcontainers.shaded.io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:255)
 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:748)
Apr 20, 2018 2:26:54 PM org.apache.catalina.loader.WebappClassLoaderBase clearReferencesThreads
WARNING: The web application [ROOT] appears to have started a thread named [testcontainers-ryuk] but has failed to stop it. This is very likely to create a memory leak. Stack trace of thread:
 java.lang.Object.wait(Native Method)
 org.testcontainers.utility.ResourceReaper.lambda$start$2(ResourceReaper.java:127)
 org.testcontainers.utility.ResourceReaper$$Lambda$99/254873768.run(Unknown Source)
 java.lang.Thread.run(Thread.java:748)
Apr 20, 2018 2:26:54 PM org.apache.catalina.loader.WebappClassLoaderBase clearReferencesThreads
WARNING: The web application [ROOT] appears to have started a thread named [testcontainers-netty-1-8] but has failed to stop it. This is very likely to create a memory leak. Stack trace of thread:
 org.testcontainers.shaded.io.netty.channel.epoll.Native.epollWait0(Native Method)
 org.testcontainers.shaded.io.netty.channel.epoll.Native.epollWait(Native.java:111)
 org.testcontainers.shaded.io.netty.channel.epoll.EpollEventLoop.epollWait(EpollEventLoop.java:231)
 org.testcontainers.shaded.io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:255)
 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:748)