Use k8s label for k8s runner testing

Differentiate k8s runner from host runner with unique label.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.forgejo/workflows/build-container.yaml

@@ -15,7 +15,7 @@ on:
 
 jobs:
   build:
-    runs-on: docker
+    runs-on: k8s
     steps:
       - name: Parse tag
         id: parse

.github/actionlint.yaml

@@ -2,3 +2,4 @@ self-hosted-runner:
   labels:
     - ubuntu-latest
     - docker
+    - k8s

argocd/manifests/forgejo-runner/secret.yaml.tpl

@@ -13,5 +13,5 @@ type: Opaque
 stringData:
   FORGEJO_URL: "https://forge.ops.eblu.me"
   RUNNER_NAME: "k8s-runner"
-  RUNNER_LABELS: "docker:docker://registry.ops.eblu.me/blumeops/forgejo-runner:v2.1.3"
+  RUNNER_LABELS: "k8s:docker://registry.ops.eblu.me/blumeops/forgejo-runner:v2.1.3"
   RUNNER_TOKEN: "{{ op://vg6xf6vvfmoh5hqjjhlhbeoaie/w3663ffnvkewbftncqxtcpeavy/runner_reg }}"