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>
diff --git a/run-unit-test-docker.sh b/run-unit-test-docker.sh
index e85661e..5a07de9 100755
--- a/run-unit-test-docker.sh
+++ b/run-unit-test-docker.sh
@@ -19,6 +19,7 @@
 FORMAT_CODE_SH="format-code.sh"
 DBUS_UNIT_TEST_PY="dbus-unit-test.py"
 DBUS_SYS_CONFIG_FILE=${dbus_sys_config_file:-"/usr/share/dbus-1/system.conf"}
+MAKEFLAGS="${MAKEFLAGS:-""}"
 
 # Timestamp for job
 echo "Unit test build started, $(date)"
@@ -70,6 +71,7 @@
 docker run --cap-add=sys_admin --rm=true \
     --privileged=true \
     -w "${WORKSPACE}" -v "${WORKSPACE}":"${WORKSPACE}" \
+    -e "MAKEFLAGS=${MAKEFLAGS}" \
     -t ${DOCKER_IMG_NAME} \
     ${WORKSPACE}/${DBUS_UNIT_TEST_PY} -u ${UNIT_TEST} \
     -f ${DBUS_SYS_CONFIG_FILE}