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