feat(sitemap): enhance sitemap generation and error handling

Add optional `updated_at` field to the Project interface for 
tracking last modification. Update base URL to use an 
environment variable for better configurability. Improve 
error handling during sitemap data fetching by logging 
errors and returning static routes as a fallback. 
Refactor change frequency and priority logic for clarity 
and maintainability.

Dockerfile

@@ -13,15 +13,9 @@ RUN npm install
 # Copy the application code
 COPY . .
 
-# Set IS_BUILD environment variable for build process
-ENV IS_BUILD=true
-
 # Build the Next.js application
 RUN npm run build
 
-# Unset IS_BUILD environment variable for runtime
-ENV IS_BUILD=false
-
 # Set environmental variable for production mode
 ENV NODE_ENV=production