feat(al2023): ensure the sandbox image is present periodically

Author: cartermckinnon

templates/al2023/runtime/rootfs/etc/systemd/system/check-sandbox-image.service

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+[Unit]
+Description=Check that the sandbox image is present in the containerd content store
+After=containerd.service
+Wants=containerd.service
+
+[Service]
+Type=oneshot
+ExecStart=/usr/bin/ctr image import --namespace k8s.io /etc/eks/pause.tar
+
+[Install]
+WantedBy=multi-user.target

templates/al2023/runtime/rootfs/etc/systemd/system/check-sandbox-image.timer

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+[Unit]
+Description=Run check-sandbox-image periodically
+Requires=check-sandbox-image.service
+
+[Timer]
+Unit=check-sandbox-image.service
+# start 1 minute after boot
+OnBootSec=1min
+# run every 5 minutes
+OnUnitActiveSec=5min
+
+[Install]
+WantedBy=timers.target