Add test to enforce volume build content

This tests ensures that the content from a dir within a build is carried
over even if VOLUME for that dir is specified in the Dockerfile.  This
test ensures this long standing functionality.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>
Merge pull request #9638 from crosbymichael/build-volumes-retain-contents

Add test to enforce volume build content