Document Linux installer and agent container updates

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Summary

• Updates Linux installation docs with the verified agent installer flow, client-only install behavior, custom controller option, and service controls.
• Updates system requirements with glibc, supervisor, downloader, and NVIDIA encode dependency details.
• Refreshes Docker agent docs for the current Ubuntu 24.04/CUDA 13 image baseline, runtime environment variables, host IP/gateway behavior, and custom image build inputs.
• Adds troubleshooting guidance for Linux installer and service installation failures.

Source verification

• Verified against accessible source and recent merged PRs in Juice-Labs/juice-oss because Juice-Labs/juice was not visible to this automation via GitHub.
• Covered codepaths: installer/install-juice.sh, docker/agent/Dockerfile, and docker/agent/root/home/juice/start-agent.sh.

Validation

• Not run: npm run build because npm is not installed in the Cloud Agent environment.