Enhance Dockerfile and entrypoint script for Kerberos initialization

- Added krb5-admin-server and updated entrypoint to ensure proper initialization of the Kerberos realm.
- Improved error handling for required environment variables in entrypoint script.
- Updated README with additional prerequisites and client configuration instructions.
- Modified env.example to remove default passwords for security.
- Enhanced run-container script to set container hostname based on KDC_HOST.

env.example

@@ -1,6 +1,6 @@
 KRB5_REALM=EXAMPLE.ORG
 KRB5_DOMAIN=example.org
 KRB5_KDC_HOST=kerberos.example.org
-KRB5_MASTER_PASSWORD=changeit
+KRB5_MASTER_PASSWORD=
 KRB5_ADMIN_PRINCIPAL=admin
-KRB5_ADMIN_PASSWORD=changeit
+KRB5_ADMIN_PASSWORD=