test: add --build option for docker-compose up in test script (#6348)

Fixes DGRAPH-2356
This PR fixes random GraphQL custom logic e2e test failures in TeamCity by requiring docker-compose to always build images before starting the containers, as the mock server in custom logic tests can change any time.

contrib/scripts/functions.sh

@@ -37,7 +37,7 @@ function restartCluster {
   fi
 
   docker ps -a --filter label="cluster=test" --format "{{.Names}}" | xargs -r docker rm -f
-  GOPATH=$docker_compose_gopath docker-compose -p dgraph -f $compose_file up --force-recreate --remove-orphans -d || exit 1
+  GOPATH=$docker_compose_gopath docker-compose -p dgraph -f $compose_file up --force-recreate --build --remove-orphans -d || exit 1
   popd >/dev/null
 
   $basedir/contrib/wait-for-it.sh -t 60 localhost:6180 || exit 1