Update healthcheck to work with PORT variable

Dockerfile

@@ -54,9 +54,10 @@ COPY --link /public ./public
 COPY --link --from=builder /app/.next/standalone ./
 COPY --link --from=builder /app/.next/static/ ./.next/static/
 
+EXPOSE 3000
+ENV PORT='3000'
+
 HEALTHCHECK --interval=12s --timeout=12s --start-period=30s \  
     CMD node ./healthcheck.js
 
-EXPOSE 3000
-ENV PORT 3000
 CMD ["node", "server.js"]