fix: gate all Docker-dependent tasks behind docker_rootless_setup

The validate.yml had an unconditional 'docker version' check, and
service.yml/prune.yml unconditionally enabled services that need
Docker running. Added when: docker_rootless_setup to:
- validate.yml: Verify rootless Docker connectivity
- service.yml: Enable and start gitea-runner service
- prune.yml: Enable and start docker-prune timer
Also made lifecycle side_effect tolerant of service start failure
since Docker daemon isn't available in molecule containers.

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ansible/roles/gitea-runner/tasks/service.yml

@@ -22,4 +22,6 @@
   environment:
     XDG_RUNTIME_DIR: "/run/user/{{ gitea_runner_uid }}"
   changed_when: true
-  when: systemd_available.stat.exists
+  when:
+    - systemd_available.stat.exists
+    - docker_rootless_setup