ci: use Jenkins harbor credential for image push auth

Jenkinsfile

@@ -93,6 +93,7 @@ spec:
     stage('Prep toolchain') {
       steps {
         container('builder') {
+          withCredentials([usernamePassword(credentialsId: 'harbor-robot', usernameVariable: 'HARBOR_USERNAME', passwordVariable: 'HARBOR_PASSWORD')]) {
             sh '''
               set -euo pipefail
               for attempt in 1 2 3 4 5; do
@@ -106,11 +107,12 @@ spec:
                 sleep $((attempt * 2))
               done
               mkdir -p /root/.docker
-            cp /docker-config/config.json /root/.docker/config.json
+              printf '%s' "${HARBOR_PASSWORD}" | docker login registry.bstein.dev -u "${HARBOR_USERNAME}" --password-stdin
             '''
           }
         }
       }
+    }
 
     stage('Compute version') {
       steps {