Add task UUID label to Kubernetes pods

[henkka]

Slightly relates to #5402 and #3344

Problem

When using the Kubernetes backend with ReadWriteOnce PersistentVolumes, volumes can only be attached to one Kubernetes worker node at a time. This creates scheduling conflicts when workflow steps are scheduled to run on different nodes (due to worker node size constraints for example).

Background

The somewhat ideal solution would be to implement Kubernetes PodAffinity and PodAntiAffinity rules directly in Woodpecker to ensure:

• All pods in the same workflow run on the same node (to share PVs)
• Pods from different workflows avoid the same node (to prevent conflicts)

However, a previous attempt to add this functionality (PR #3345) was declined as there's different views how the root problem should be resolved. In our opinion, #3366 sounds like a good approach, but at this moment we cannot afford to wait for implementing such a major change.

Proposed Solution

Instead of modifying Woodpecker's core scheduling logic, this PR enables external solutions by adding a woodpecker-ci.org/task-uuid label to each pod. This label:

• Contains the workflow/task UUID (identical for all pods in the same workflow)
• Allows external tools like Kubernetes MutatingAdmissionWebhooks to inject appropriate affinity rules to the pods scheduled by Woodpecker Agent
• Provides a clean separation of concerns between Woodpecker and cluster-specific scheduling policies

TL;DR: We figured this approach is the path of least resistance that allows Woodpecker CI users to overcome the background issue without big changes to Woodpecker CI project itself.

[woodpecker-bot]

Surge PR preview deployment succeeded. View it at https://woodpecker-ci-woodpecker-pr-5544.surge.sh

[codecov]

Codecov Report

❌ Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 26.33%. Comparing base (7707e84) to head (b053436).

Files with missing lines	Patch %	Lines
pipeline/backend/kubernetes/pod.go	80.00%	2 Missing ⚠️
pipeline/backend/kubernetes/kubernetes.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5544   +/-   ##
=======================================
  Coverage   26.32%   26.33%           
=======================================
  Files         405      405           
  Lines       29028    29031    +3     
=======================================
+ Hits         7641     7644    +3     
  Misses      20688    20688           
  Partials      699      699           

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[xoxys]

Thanks! I like this approach as a pragmatic way to solve the current issue.

[henkka]

Thank you for the really quick turnaround on this pull request! We're amazed by how well this project is run and how easily we can contribute to it.