chore(Jenkinsfile) switch to private registry and only build for arm64

- Use private registry: jenkins-infra/helpdesk#4769
- No need to build for `amd64` nowadays (less data to pull/push and less agents to use)

Author: dduportal

Jenkinsfile_k8s

@@ -1 +1,4 @@
-buildDockerAndPublishImage('plugin-site-issues', [targetplatforms: 'linux/amd64,linux/arm64'])
+buildDockerAndPublishImage('plugin-site-issues', [
+  publishToPrivateAzureRegistry: true,
+  targetplatforms: 'linux/arm64',
+])