jenkins: Add default imagePullSecrets for jobs

Setting the `imagePullSecrets` property on the default service account
for the *jenkins-jobs* namespace allows jobs to run from private
container images automatically, without additional configuration in the
pipeline definitions.

jenkins/jenkins.yaml

@@ -16,6 +16,15 @@ metadata:
   name: jenkins
   namespace: jenkins
 
+---
+apiVersion: v1
+kind: ServiceAccount
+metadata:
+  name: default
+  namespace: jenkins-jobs
+imagePullSecrets:
+- name: imagepull-gitea
+
 ---
 apiVersion: v1
 kind: PersistentVolumeClaim