feat(docker-entrypoint.sh): add shebang and implement envsubst to ena…

…ble environment variable substitution in nginx html files

react-frontend/docker-entrypoint.sh

@@ -1,3 +1,5 @@
+#!/bin/env bash
+
 for file in /usr/share/nginx/html/*; do
     if [ -f "$file" ]; then
         envsubst < "$file" > "$file.tmp" && mv "$file.tmp" "$file"