There are a number of issues with this. Firstly, it needs to copy the
parent directories to make a heirarchy in the .zuul-siblings
directory. The current "cp -r" was only copying the final directory.
Switch into the source directory and use "--parent" to do this.
Also, it should be copying into the context dir. Add the
{{ item.context }} to the path where appropriate.
Make new testing image that copies in files from the siblings.
Because COPY will fail if the sources aren't there, this is like an
assert that we copied it correctly.
Change-Id: I9f3b0a1f71d20cf7511f224648dd2fa51a039015
599954715e