run-unit-test-docker.sh: Pass MAKEFLAGS into container

This allows us to set options like "-j$(nproc)" or "V=1" as required.

Tested: Exported MAKEFLAGS and ran run-unit-test-docker.sh, confirmed
verbose output, concurrent jobs executed.

Change-Id: Ib640c22b034cdf673a1902e91f8a47395033fc98
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
1 file changed