Only singularity pull 3 containers at a time

.gitlab-ci.yml

@@ -33,7 +33,7 @@ img_cache:
   stage: singularity
   script:
     - mkdir -p ${dir}cache/
-    - cat nextflow.config | grep -oP "container = \K.*" | tr -d "'" | sort | uniq | xargs -P 5 -I {} singularity pull --dir ${dir} 'docker://'{} || true
+    - cat nextflow.config | grep -oP "container = \K.*" | tr -d "'" | sort | uniq | xargs -P 3 -I {} singularity pull --dir ${dir} 'docker://'{} || true
 
 collect:
   stage: versions