Adds some basic READMEs to some of the test images

Clarifies the use/purpose of some of the test images for easier
discovery/use in the future.

test/images/ipc-utils/README.md

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+# ipc-utils
+
+This container will be used to exercise the HostIPC functionality in
+e2e-node tests.
+
+The version of `ipcs` shipped in busybox performs operations that get
+blocked by SELinux on hosts where it is enabled. The version of `ipcs`
+in util-linux does not perform those operations, rather it checks
+whether the /proc files it needs are available and proceeds to read
+from them directly.
+
+Using `ipcs` from util-linux makes these tests pass, even when running
+under SELinux enabled, so let's use them here.

test/images/jessie-dnsutils/README.md

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+# jessie-dnsutils
+
+DNS utils but implemented in a different OS to have some basic check across OSes.
+See issue #10161 for original issue and discussion.

test/images/metadata-concealment/README.md

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+# metadata-concealment
+
+Runs various checks against the GCE metadata API. If the checks pass, the job is run to completion. It fails and stops at the first error encountered.
+
+Only valid if using the GCE provider.

test/images/nautilus/README.md

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+# nautilus
+
+A basic file server that serves an image. The underlying base image does most
+of the work but we load a specific image when building this one so that we
+can test that a pod's image really changed. Often used to contrast against
+the kitten image which loads/serves a different image.

test/images/nonewprivs/README.md

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+# nonewprivs
+
+A simple go app that prints the UID of the process running to test security context features.

test/images/nonroot/README.md

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+# nonroot
+
+A trivial image that just builds in the default user ID to confirm how it interacts with other security settings.

test/images/redis/README.md

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+# redis
+
+Image to run Redis for our guestbook application.