Paralel execution inside a Bash script

Summary

I try to use & to run multiple commands in parallel in gitlabci. However I receive an error code and no description on what had happened

Steps to reproduce

Launch a bash scirpt with & in .gitlabci

.gitlab-ci.yml

before_script:
  - apk update
  - apk add --update bash 


custom_deploy:
  except:
    refs:
      - develop
      - useracceptance
      - master
  stage: deploy
  when: manual
  script:
    - (cd deploy && ./deploy_test.sh ${ENV} ${IMAGE_TAG})

deploy_test.sh

#!/bin/bash
 N=10 #Maximum paralel launches
  for f in config/"$DEPLOYMENT_ENV"/$ENV/*; do
     echo "Launching -> $f"
     sceptre --var-file  "$f" --var "imageversion=$IMG_VERSION" launch "$DEPLOYMENT_ENV"/ecs-service -y &
    (( ++count % N == 0)) && wait
    done

Actual behavior

Launching -> config/CI/MA/fcid
Launching -> config/CI/MA/fcid2
Launching -> config/CI/MA/mvp
Launching -> config/CI/MA/mvp3
ERROR: Job failed: exit code 1

Expected behavior

Script should launch multiple environments in paralel

Environment description

I'm using Gitlab enterprise hosted on https://gitlabhost.com/

Used GitLab Runner version

Running withgitlab-runner 10.6.0-rc1 (0a9d5de9)
Using Docker executor with image docker:latest