Fix incorrect Docker image references in documentation

[wgzhao]

The current documentation incorrectly refers to non-existent Docker images 'apache/doris.fe-ubuntu' and 'apache/doris.be-ubuntu'. The correct image format is 'apache/doris:fe-' and 'apache/doris:be-', which are regularly updated in the Docker registry.

Updated the guide to reflect the actual image naming convention.

Versions

• [X] dev
• [X] 3.0
• [x] 2.1
• [ ] 2.0

Languages

• [x] Chinese
• [x] English

Docs Checklist

• [ ] Checked by AI
• [ ] Test Cases Built