Add .env file creation in frontend Dockerfile

frontend/Dockerfile

@@ -13,6 +13,9 @@ RUN npm ci
 # Copy entire project
 COPY . .
 
+# Create a .env file before build
+RUN echo "NEXT_PUBLIC_HCAPTCHA_SITE_KEY=$HCAPTCHA_SITE_KEY" > .env
+
 # Build the Next.js app
 RUN npm run build