Feature: add HARP/ExApps support

[Kakise]

Description of the change

I tried to add a StatefulSet with HARP to run exapps on kubernetes.
My tests were successful.

StatefulSet code:

apiVersion: v1
kind: ConfigMap
metadata:
  name: docker-daemon-config
data:
  daemon.json: |
    {
      "memory": "512m"
    }
---
apiVersion: apps/v1
kind: StatefulSet
metadata:
  labels:
    app: clouduvet
  name: clouduvet-harp
spec:
  serviceName: harp
  replicas: 1
  selector:
    matchLabels:
      app: clouduvet
  template:
    metadata:
      labels:
        app: clouduvet
    spec:
      volumes:
        - name: docker-config
          configMap:
            name: docker-daemon-config
      restartPolicy: Always
      securityContext:
        fsGroup: 1000
      containers:
        - name: appapi-harp
          image: ghcr.io/nextcloud/nextcloud-appapi-harp:release
          imagePullPolicy: Always
          env:
            - name: DOCKER_HOST
              value: unix:///run/user/1000/docker.sock
            - name: HP_SHARED_KEY
              valueFrom:
                secretKeyRef:
                  name: harp-shared-key
                  key: shared-key
            - name: NC_INSTANCE_URL
              value: "http://nextcloud.nextcloud.svc.cluster.local"
          resources:
            requests:
              cpu: "250m"
              memory: "512Mi"
            limits:
              cpu: "250m"
              memory: "512Mi"
          #securityContext:
          #  privileged: true
#---
#apiVersion: v1
#kind: Pod
#metadata:
#  name: appapi-harp
#  labels:
#    app: appapi-harp
#spec:
#  restartPolicy: Always
#  hostNetwork: false
#  containers:
#    - name: appapi-harp
#      image: ghcr.io/nextcloud/nextcloud-appapi-harp:release
#      imagePullPolicy: IfNotPresent
#      env:
#        - name: HP_SHARED_KEY
#          valueFrom:
#            secretKeyRef:
#              name: harp-shared-key
#              key: shared-key
#        - name: NC_INSTANCE_URL
#          value: "http://nextcloud.nextcloud.svc.cluster.local"
#      ports:
#        - containerPort: 8780
#        - containerPort: 8782
#      volumeMounts:
#        - name: docker-sock
#          mountPath: /var/run/docker.sock
#        - name: certs
#          mountPath: /certs
#  volumes:
#    - name: docker-sock
#      hostPath:
#        path: /var/run/docker.sock
#        type: Socket
#    - name: certs
#      hostPath:
#        path: /absolute/path/to/certs  # Change to absolute path on host
#        type: Directory

It allowed me to expose a docker rootless using harp. I put that here mostly so others may be able to reference it, feel free to use it :)
You also need to change the service and the ingress.

Benefits

ExApps support.

Possible drawbacks

The StatefulState approach is losely based on what I've done for gitlab runners on my cluster, it might not be the best approach.
I cannot guarantee that the manifest I provided can be used across every clusters.

Additional information

I am willing to create a pull request for this change. However, I'd appreciate some guidance on how to expose the docker socket of the host node in a manner that is compatible with a maximum of setups.

[pschichtel]

I wonder if something like kubedock would work nicely to get ExApps to work without having to mess with docker in kubernetes.

On your proposal: From reading the docs it seems that HaRP itself is a stateless application that could be deployed as a Deployment and replicated, right? Also it doesn't seem to carry a docker daemon itself and the DOCKER_HOST variable doesn't seem to do anything.