jenkins: Add PVC for Buildroot jobs

Buildroot jobs really benefit from having a persistent workspace volume
instead of an ephemeral one.  This way, only the packages, etc. that
have changed since the last build need to be built, instead of the whole
toolchain and operating system.

jenkins/kustomization.yaml

@@ -10,6 +10,7 @@ resources:
 - secrets.yaml
 - iscsi.yaml
 - ssh-host-keys
+- workspace-volume.yaml
 
 patches:
 - patch: |

jenkins/workspace-volume.yaml

@@ -0,0 +1,14 @@
+apiVersion: v1
+kind: PersistentVolumeClaim
+metadata:
+  name: buildroot
+  labels:
+    app.kubernetes.io/name: buildroot
+    app.kubernetes.io/component: jenkins
+spec:
+  accessModes:
+  - ReadWriteOnce
+  resources:
+    requests:
+      storage: 100Gi
+  storageClassName: synology-iscsi