docs(PL-391): add guide for installing extra JDK versions on self-hosted runners

[arcappcircle]

Summary

Adds a customer-facing guide for installing a JDK version that is not preinstalled on a self-hosted Linux runner, using Java 18 as the worked example.

Context is PL-391. The issue originally asked to preinstall Java 18 on the runner image; the outcome of the discussion there is that we do not ship non-LTS / EOL tooling on runner images, and a customer needing one performs the install on their own self-hosted runner. This page is the procedure that makes that answer actionable. It is the public counterpart to the internal LTS/EOL policy in ac-doc-internal#203.

Changes

• New page self-hosted-appcircle/self-hosted-runner/configure-runner/custom-jdk-versions.md:
  • Install the JDK with SDKMAN, including the local-candidate path for versions that SDKMAN has dropped from its catalog (Java 18 is no longer listed for any vendor).
  • Declare JAVA_HOME_<major>_X64 in /etc/systemd/system/io.appcircle.runner.service. Adding the export to ~/.bashrc alone does not work, because the generated startup.sh does not source it at runtime, and the unit template only covers 8, 11, 17 and 21.
  • systemctl daemon-reload before restarting, since ac-runner service -c restart only stops and starts the service.
  • Select the JDK from a workflow with a Custom Script step, because the Select Java Version dropdown is a fixed list.
  • Docker-based runners: the mechanism is identical since systemd runs as PID 1 inside the container, with three deltas (run the steps via docker exec, HOME is always /root, and container recreation is the one lifecycle event that loses the JDK). Also states explicitly that docker run -e, --env-file, Compose environment: and Dockerfile ENV do not reach the runner.
• Link added from infrastructure/android-build-infrastructure.md, in the ## Java Version section right after the Select Java Version sentence, via the ContentRef component already imported on that page.
• Link added from the Configure Runner index.md for discoverability.

Review notes

• Please double-check the Azul download URL and the zulu18.32.13-ca-jdk18.0.2.1-linux_x64 archive name in Step 2 against Azul's current download page. The build string comes from the PL-391 discussion, not from a verified download.
• The volume names in the Docker docker run example are illustrative. We do not publish a runner image, so there is no canonical mount layout to match.

🤖 Generated with Claude Code

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Created on behalf of ilhan@appcircle.io via Arc.

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