Set HCAPTCHA key in .env file during Docker build

This change adds a step to create a `.env` file with the HCAPTCHA site key during the Docker build process. It ensures the environment variable is available for use within the container.

frontend/Dockerfile

@@ -17,6 +17,9 @@ COPY . .
 ARG HCAPTCHA_SITE_KEY
 ENV NEXT_PUBLIC_HCAPTCHA_SITE_KEY=${HCAPTCHA_SITE_KEY}
 
+# Create a .env file before build
+RUN echo "NEXT_PUBLIC_HCAPTCHA_SITE_KEY=$HCAPTCHA_SITE_KEY"
+
 # Build the Next.js app
 RUN npm run build