run-unit-test-docker: Allow overriding workdir
run-unit-test-docker: Allow overriding workdir

There are some cases where we want to be able to override the location
of the build workspace inside the container and decouple the location
from the host location. This preserves the current behavior if
DOCKER_WORKDIR is not set.

Change-Id: I885f132e9da06ce9f3c1ddd9dfc7c69b9fb08cd4
Signed-off-by: William A. Kennington III <30fff5198f4d10ca1117e5f3ec82f3b4b8900d81@google.com>

openbmc/openbmc-build-scripts,openbmc/openbmc-build-scripts