fix(groovy): preserve the configured Script task runner

[kimsanhaa]

What changes are being made and why?

Groovy Script replaces the configured task runner with a new Docker runner, discarding settings such as a custom Podman socket and an explicit container user.

Keep the selected runner and its options. For Docker, default to root only when no user is configured, matching Groovy Commands. Also preserve an explicit user in the deprecated docker options.

Closes #333.

How the changes have been QAed?

./gradlew :plugin-script-groovy:test: 15 tests passed, including Docker execution and the Groovy flow test. The new runner regression tests fail in four cases against the original implementation and all pass with the fix.

./gradlew :plugin-script-groovy:spotlessCheck reports existing formatting violations in the unchanged CommandsTest.java; neither changed file has a formatting violation.

Example for a worker using a rootless Podman socket (adjust the socket path):

id: groovy_custom_runner
namespace: company.team

tasks:
  - id: hello
    type: io.kestra.plugin.scripts.groovy.Script
    taskRunner:
      type: io.kestra.plugin.scripts.runner.docker.Docker
      host: unix:///run/user/1000/podman/podman.sock
    script: |
      println "Hello from Groovy!"

The custom socket is covered by the runner configuration test. A live Podman instance and the Kestra UI were not used for local QA.

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