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.

Dockerfile

@@ -4,7 +4,8 @@ RUN apt-get update && \
     DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
         krb5-kdc \
         krb5-admin-server \
-        krb5-config && \
+        krb5-config \
+        tini && \
     rm -rf /var/lib/apt/lists/*
 
 COPY entrypoint.sh /entrypoint
@@ -13,4 +14,4 @@ RUN chmod +x /entrypoint
 
 EXPOSE 88/tcp 88/udp 464/tcp 464/udp 749/tcp
 
-ENTRYPOINT ["/entrypoint"]
+ENTRYPOINT ["tini", "--", "/entrypoint"]