updated container entrypoint to envsubst the index.html as well

entrypoint.sh

 #!/bin/sh
 # Replace env vars in JavaScript files
 
-echo "Replacing env vars in JS"
+echo "Replacing env vars in JS/HTML"
 cp -r /var/templates/* /var/www/html/
 
 echo "Set API Url to: $MAT_API_HOST_PLACEHOLDER"
@@ -17,5 +17,11 @@ do
   rm $file.tmpl.js
 done
 
+echo "Processing index.html ...";
+cp /var/www/html/index.html /var/www/html/index.html.tmpl.html
+envsubst '$MAT_API_HOST_PLACEHOLDER' < /var/www/html/index.html.tmpl.html > /var/www/html/index.html
+rm /var/www/html/index.html.tmpl.html
+
+cat /var/www/html/index.html
 echo "Starting Nginx"
 nginx -g 'daemon off;'