Image as env var. (#17828)

.gitlab-ci/scripts/test/postgres.sh

@@ -21,7 +21,7 @@ docker run -d -it --rm --name "${CONTAINER_SERVER}" --net ${DOCKER_NETWORK} \
   --env MM_SQLSETTINGS_DRIVERNAME=postgres \
   -v $CI_PROJECT_DIR:/mattermost-server \
   -w /mattermost-server \
-  ${CI_REGISTRY}/mattermost/ci/images/mattermost-build-server:20201119_golang-1.15.5 \
+  $IMAGE_BUILD_SERVER \
   bash -c "ulimit -n 8096; make test-server$RACE_MODE BUILD_NUMBER=$CI_COMMIT_REF_NAME-$CI_COMMIT_SHA TESTFLAGS= TESTFLAGSEE=" \
   bash -c scripts/diff-email-templates.sh
 mkdir -p logs