restic: Add restic-prune CronJob

This CronJob schedules a periodic run of `restic forget`, which deletes
snapshots according to the specified retention period (14 daily, 4
weekly, 12 monthly).

This task used to run on my workstation, scheduled by a systemd timer
unit.  I've kept the same schedule and retention period as before.  Now,
instead of relying on my PC to be on and awake, the cleanup will occur
more regularly.  There's also the added benefit of getting the logs into
Loki.

restic/kustomization.yaml

@@ -0,0 +1,50 @@
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+
+namespace: restic
+
+labels:
+- pairs:
+    app.kubernetes.io/instance: restic
+  includeSelectors: true
+- pairs:
+    app.kubernetes.io/part-of: restic
+  includeTemplates: true
+
+resources:
+- namespace.yaml
+- network-policy.yaml
+- restic-prune.yaml
+- secrets.yaml
+- ../dch-root-ca
+
+configMapGenerator:
+- name: restic-env
+  envs:
+  - restic.env
+
+patches:
+- patch: |-
+    apiVersion: batch/v1
+    kind: CronJob
+    metadata:
+      name: restic-prune
+    spec:
+      jobTemplate:
+        spec:
+          template:
+            spec:
+              containers:
+              - name: restic-prune
+                env:
+                - name: RESTIC_CACERT
+                  value: /run/dch-ca/dch-root-ca.crt
+                volumeMounts:
+                - mountPath: /run/dch-ca
+                  name: dch-ca
+                  readOnly: true
+              volumes:
+              - name: dch-ca
+                configMap:
+                  name: dch-root-ca
+