Update Dockerfile to patch OS vulnerabilities and update README

[jimmychiuuuu]

1. Dockerfile Updates:

• Added apt update, apt -y upgrade, and apt -y autoremove to patch base OS vulnerabilities and clean up dependencies.
• Addresses container image vulnerabilities.

2. README Enhancements:

• Clarified support for vTPM/SEV, AMD SEV-SNP, and Intel TDX.
• Updated prerequisites to specify requirements for AMD SNP and Intel TDX enabled machines.

[kongoshuu]

Please remove the link in the commit message since it's an internal link that we should avoid in open source environment, and avoid referring to internal terms.