build-unit-test-docker: use docker stagename everywhere

In some cases the script used a 'stagename' as the temporary docker
image used to create dependencies.  Use this throughout for
consistency.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I8b2d8624747b8a36630dd27f6aee32ddf10defa7
1 file changed