Add DL agent

[angelini]

A design for the agent and the K8S tooling necessary to run it locally with Docker desktop.

$ make setup-k8s
$ make deploy-k8s

Connect to the demo sandbox pod:

$ kubectl --context docker-desktop -n dateilager exec -it dl-sandbox-bbfb85548-r92vg -- bash
bash-5.2$ curl dl-agent.dateilager.svc.cluster.local:8080/healthz
{"status":"OK"}
bash-5.2$ curl -XPOST -H 'Content-Type: application/json' -d "{\"uid\":\"${K8S_CONTAINER_ID}\", \"volume\": \"appdir\"}"  dl-agent.dateilager.svc.cluster.local:8080/link_cache

And it is currently failing with:

ln /home/main/varlib/cni/cache/results/cni-loopback-00d359d08d903c13c8e918d44f02db2f2b81084e298d39bbd0818392d9f6fdcc-eth0 /tmp/dl_cache_k8s/kubelet/pods/5c7ee044-ba3a-4b8a-b25b-b192d4f85eec/volumes/kubernetes.io~projected/appdir/dl_cache/cni/cache/results/cni-loopback-00d359d08d903c13c8e918d44f02db2f2b81084e298d39bbd0818392d9f6fdcc-eth0: link /home/main/varlib/cni/cache/results/cni-loopback-00d359d08d903c13c8e918d44f02db2f2b81084e298d39bbd0818392d9f6fdcc-eth0 /tmp/dl_cache_k8s/kubelet/pods/5c7ee044-ba3a-4b8a-b25b-b192d4f85eec/volumes/kubernetes.io~projected/appdir/dl_cache/cni/cache/results/cni-loopback-00d359d08d903c13c8e918d44f02db2f2b81084e298d39bbd0818392d9f6fdcc-eth0: invalid cross-device link