Document Linux agent installer and container setup

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Summary

• Adds Linux agent installer usage to the installation guide, including service install behavior, quoted pool names, custom controller configuration, and pass-through agent run arguments.
• Expands Docker agent docs with Docker Compose and CLI examples, runtime environment variables, update guidance, and current custom-build prerequisites.
• Fixes the Kubernetes quickstart secret name/key guidance and updates Docker connectivity troubleshooting for UDP port publishing and JUICE_HOST_IP.

Source coverage

• Verified against accessible upstream Juice-Labs/juice-oss changes and source paths: installer/install-juice.sh, docker/agent/Dockerfile, docker/agent/README.md, charts/agent/values.yaml, and charts/agent/templates/_agent.yaml.
• Relevant recent upstream PRs reviewed: pool-name quoting, service install command changes, Docker agent rebuild on Ubuntu 24.04, and custom controller installer support.

Validation

• git diff --check
• npm run build (passes; existing site-wide broken edit-link warnings and Browserslist stale data warning remain)

Note

• Direct gh access to Juice-Labs/juice returned a repository access error in this automation environment, so source verification used the accessible Juice-Labs/juice-oss repository.