chore: migrate to pnpm for package management and update CI configuration

- Replaced npm with pnpm in the CI configuration and Dockerfile for improved dependency management.
- Updated package.json to specify pnpm as the package manager.
- Modified installation instructions in README.md to reflect the use of pnpm.
- Enhanced the build process in the Dockerfile to utilize pnpm for installing dependencies and building the application.

package.json

@@ -2,6 +2,7 @@
   "name": "free-nextadmin-nextjs",
   "version": "1.2.1",
   "private": true,
+  "packageManager": "pnpm@10.18.0",
   "scripts": {
     "dev": "next dev",
     "dev:e2e": "next dev -p 3000",