add_gtm_script.sh

#!/bin/bash
if [[ -n "${GTM_CONTAINER_ID}" ]]; then
  CODE="<script>(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','$GTM_CONTAINER_ID');</script>"
  sed -i "s#</head>#${CODE}</head>#g" /usr/share/nginx/html/index.html

  CODE="<noscript><iframe src=\"https://www.googletagmanager.com/ns.html?id=${GTM_CONTAINER_ID}\" height="0" width="0" style=\"display:none;visibility:hidden\"></iframe></noscript>"
  sed -i "s#</body>#${CODE}</body>#g" /usr/share/nginx/html/index.html
fi