feat(docker): unify dynamic PORT across Dockerfile, compose, and dev server

Dockerfile

@@ -5,9 +5,10 @@ WORKDIR /app
 # Dynamic build arguments & environment variables
 ARG SITE_DOMAIN
 ARG SITE_URL
+ARG PORT=8080
 ENV SITE_DOMAIN=$SITE_DOMAIN
 ENV SITE_URL=$SITE_URL
-ENV PORT=8080
+ENV PORT=$PORT
 ENV NODE_ENV=production
 
 # Install dependencies
@@ -20,8 +21,8 @@ COPY . .
 # Build static production assets
 RUN npm run build
 
-# Expose web server port
-EXPOSE 8080
+# Expose web server port dynamically
+EXPOSE ${PORT}
 
 # Serve site with npm
 CMD ["npm", "start"]