build-unit-test-docker: Generate depcache in parallel

Depcache lookup is a fairly slow process when done serially, because it
has to query github one repo at a time. We can greatly speed this up by
parallelizing the whole thing, and the sort at the end of stability.

Tested:
    Rebuilds work as expected when versions change and when new packages
    are added. Does not rebuild in the case of no changes being made.

Change-Id: Iefeb8aedef1255425c171bd353ff6d56135af87f
Signed-off-by: William A. Kennington III <wak@google.com>
1 file changed