packer.json

{
    "variables": {
        "account_file": "{{ env `GOOGLE_APPLICATION_CREDENTIALS` }}"
    },

   "builders": [
       {
           "type": "googlecompute",
           "project_id": "GCP_PROJECT_ID",
           "source_image_family": "ubuntu-2004-lts",
           "account_file": "{{ user `account_file` }}",
           "zone": "us-central1-a",
           "ssh_username": "root",
           "image_name": "portfolio-web-app-{{timestamp}}",
           "image_description": "Web server for portfolio web app.",
           "image_storage_locations": ["us-central1"],
           "image_labels": {
               "developer": "rohan",
               "team": "cloud"
           },
           "network": "packer-vpc",
           "subnetwork": "packer-vpc",
           "service_account_email": "packer-sa@gcp_project.iam.gserviceaccount.com"
       }
   ],
   "provisioners": [
       {
           "type": "shell",
           "inline": ["mkdir -p /var/www/html"]
       },
       {
           "type": "file",
           "source": "public/",
           "destination": "/var/www/html"
       },
       {
           "type": "shell",
           "script": "deploy.sh"
       }
   ],
   "post-processors": [
       {
           "type": "vagrant"
       },
       {
           "type": "compress",
           "output": "portfolio-web-app-vagrant.tar.gz"
       },
       {
            "type": "googlecompute-export",
            "paths": [
                "gs://gcs_bucket/portfolio-web-app-vagrant.tar.gz"
            ],
            "keep_input_artifact": true
      }
   ]
}

script.sh

#! /bin/bash
#prepare for installs
apt-get -f autoremove -y
apt-get update -y
apt-get upgrade -y

#install nginx
apt install nginx -y

#enable ufw
ufw allow 'Nginx HTTP'
ufw enable

#configure
chown -R $USER:$USER /var/www/html
systemctl start nginx
systemctl enable nginx