Add runtime field to Docker agent template container settings

[Ross-s]

Adds a runtime field to DockerTemplateBase so users can specify a
container runtime (e.g. sysbox-runc, nvidia) per agent template,
without having to set a default runtime globally in /etc/docker/daemon.json.

Closes #1031

What changed

• Added runtime field to DockerTemplateBase
• Wired into HostConfig.withRuntime(runtime) when creating containers
• Added <f:entry> for Runtime in config.jelly (after Security
  Options), with inline help
• Added unit tests

Why

The HostConfig API in docker-java already supports .withRuntime(String)
but it was never exposed in the plugin UI. This is a minimal, additive
change with no impact on existing behaviour — the field defaults to null
and is only applied when non-empty.

Use case driving this: running Jenkins agents with
Nestybox Sysbox for secure
Docker-in-Docker without --privileged, which requires --runtime=sysbox-runc.

Testing done

• Unit test: DockerTemplateBaseTest#testRuntimeField — verifies field
  is stored and retrieved correctly
• Manual: provisioned a sysbox-runc agent via Jenkins Docker Build Cloud;
  docker info inside the agent confirms DinD is functional

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed

[Copilot]

Pull request overview

This PR adds per-template Docker runtime selection to the Jenkins Docker agent configuration so individual agent templates can request a non-default container runtime without changing the Docker daemon’s global default. It extends DockerTemplateBase, exposes the setting in the Jenkins UI, and applies it when building the container HostConfig.

Changes:

• Added a new runtime field to DockerTemplateBase with databinding support and container creation wiring via HostConfig.withRuntime(...).
• Added a new Runtime entry and inline help to the Docker template configuration UI.
• Added unit coverage for applying the runtime during fillContainerConfig(...).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/main/java/com/nirima/jenkins/plugins/docker/DockerTemplateBase.java	Adds the runtime property, setter/getter, and applies it to Docker container host configuration.
src/main/resources/com/nirima/jenkins/plugins/docker/DockerTemplateBase/config.jelly	Exposes the new runtime field in the Jenkins agent template configuration form.
src/main/resources/com/nirima/jenkins/plugins/docker/DockerTemplateBase/help-runtime.html	Documents the new runtime option for users in the UI help.
src/test/java/com/nirima/jenkins/plugins/docker/DockerTemplateBaseTest.java	Adds tests for runtime handling in container configuration assembly.

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