Update Juice Linux installer and Docker agent docs

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Summary

• Updated the Linux installation guide with the current install-script flow, including client-only installs when INSTALL_JUICE_POOL is omitted.
• Documented installer environment variables including INSTALL_JUICE_CONTROLLER, version/user/service-start controls, and service argument forwarding.
• Refreshed Docker agent docs with Compose usage, direct docker run --gpus all, runtime environment variables, update guidance, and custom build inputs.
• Updated Linux system requirements to call out the glibc 2.27 minimum.

Codepaths Covered

• installer/install-juice.sh in Juice-Labs/juice-oss PRs Update index.md #6, Update cli-login.md. Correct m2m documentation, make it consistent #7, and Update Manually Managing Sessions #11.
• docker/agent/Dockerfile, docker/agent/README.md, and docker/agent/root/home/juice/start-agent.sh in Juice-Labs/juice-oss PR Add metrics to documentation #10.

Knowledge Gaps Addressed

• Clarifies when the Linux installer creates an agent service versus installing client tools only.
• Documents custom-controller setup for private/self-hosted controller deployments.
• Makes Docker agent setup scannable with required token, pool, host IP, GPU exposure, and update steps.

Verification

• npm run build passed after npm ci.
• Build emitted existing site-wide broken-link warnings for /juice-docs/edit?path=... edit links.

Source Access Note

• Direct gh and web access to Juice-Labs/juice returned 404 from this automation environment, so the documented behavior was verified against the accessible Juice-Labs/juice-oss source and merged PR diffs linked from the existing docs.