docs: refresh linux installer and docker agent guidance from recent upstream changes

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Summary

Updated existing docs to reflect recent merged behavior in Juice-Labs/juice-oss installer and Docker agent codepaths.

Docs Updated

• docs/juice/user-guide/setup/installation.md
  • Added one-step Linux installer guidance using get.juicelabs.co
  • Documented current installer variables and defaults (INSTALL_JUICE_TOKEN, optional INSTALL_JUICE_POOL, INSTALL_JUICE_CONTROLLER, INSTALL_JUICE_VERSION)
  • Added Linux installer troubleshooting table for common failure modes
• docs/juice/user-guide/cli-app/reference/index.md
  • Corrected agent service install [pool-id] parameter docs to mark pool-id as optional
• docs/juice/user-guide/docker/docker-agent.md
  • Updated custom build workflow and prerequisites
  • Added explicit requirement for Video_Codec_SDK_<version>.zip and juice-gpu-linux.tar.gz
  • Added common build/runtime pitfalls and fixes

Verified Upstream Codepaths Covered

• installer/install-juice.sh
  • optional service setup when INSTALL_JUICE_POOL is omitted
  • custom controller support via INSTALL_JUICE_CONTROLLER
  • dependency validation behavior and glibc floor checks
  • service install error behavior (Could not install service)
• docker/agent/Dockerfile
  • required local Video_Codec_SDK_<version>.zip
  • required juice-gpu-linux.tar.gz in build context
• docker/agent/root/home/juice/start-agent.sh
  • required runtime env vars (JUICE_TOKEN, JUICE_POOL, JUICE_HOST_IP)

Knowledge Gaps Addressed

• Clarified installer intent and architecture (binary-only install vs service registration)
• Clarified recently changed installer parameters and constraints
• Added actionable troubleshooting steps for common installation/build failures

Notes

Juice-Labs/juice was not accessible to this automation token (404), so updates were verified against Juice-Labs/juice-oss, which contains the installer and Docker agent source used for these docs areas.