[scripts] Fix call to docker-pull script

scripts/before_build.sh

@@ -15,4 +15,5 @@ conda activate beat-web
 run_cmd buildout
 
 # pull docker images
-run_cmd ${SCRIPTS_DIR}/docker_pull.sh
+thisdir=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+run_cmd ${thisdir}/docker_pull.sh