01-yarn.config

 files: 
  /opt/elasticbeanstalk/hooks/appdeploy/pre/09_yarn.sh: 
    content: |-
        #!/usr/bin/env bash
        set -xe
        
        EB_SCRIPT_DIR=$(/opt/elasticbeanstalk/bin/get-config container -k script_dir)
        EB_APP_STAGING_DIR=$(/opt/elasticbeanstalk/bin/get-config container -k app_staging_dir)
        EB_APP_USER=$(/opt/elasticbeanstalk/bin/get-config container -k app_user)
        EB_SUPPORT_DIR=$(/opt/elasticbeanstalk/bin/get-config container -k support_dir)
        
        . $EB_SUPPORT_DIR/envvars
        . $EB_SCRIPT_DIR/use-app-ruby.sh
        
        # Install nodejs
        echo "install nodejs"
        curl --silent --location https://rpm.nodesource.com/setup_12.x | sudo bash -
        yum -y install nodejs
        echo "install yarn"
        # install yarn
        wget https://dl.yarnpkg.com/rpm/yarn.repo -O /etc/yum.repos.d/yarn.repo;
        yum -y install yarn;
        
        # yarn install
        cd $EB_APP_STAGING_DIR
        yarn
        
        # mkdir /home/webapp
        mkdir -p /home/webapp
        chown webapp:webapp /home/webapp
        chmod 700 /home/webapp
    group: root
    mode: "000755"
    owner: root

Second try