Fixes the new testcase for PR #8230

This new version makes sure that the same context is used for the two
builds run in the test.  If you don't use the same build then about 1/2 the
time the file copied into the container will look like a different file,
probably due to timestamp differences.  But reusing the same context we
re-use the same file on disk and therefore avoid the change in timestamps,
and we use the cache on the 2nd build.

Signed-off-by: Doug Davis <dug@us.ibm.com>