document how to run kind in a kubernetes pod

[BenTheElder]

NOTE: We do NOT recommend doing this if it is at all avoidable. We don't have another option so we do it ourselves, but it has many footguns.

xref: #284 additionally these mounts are known to be needed:

    volumeMounts:
      # not strictly necessary in all cases
      - mountPath: /lib/modules
        name: modules
        readOnly: true
      - mountPath: /sys/fs/cgroup
        name: cgroup

   volumes:
    - name: modules
      hostPath:
        path: /lib/modules
        type: Directory
    - name: cgroup
      hostPath:
        path: /sys/fs/cgroup
        type: Directory

thanks to @maratoid

/kind documentation /priority important-longterm

We probably need a new page in the user guide for this.

EDIT: Additionally, for any docker in docker usage the docker storage (typically /var/lib/docker) should be a volume. A lot of attempts at using kind in Kubernetes seem to miss this one. Typically an emptyDir is suitable for this.

EDIT2: you also probably want to set a pod DNS config to some upstream resolvers so as not to have your inner cluster pods trying to talk to the outer cluster's DNS which is probably on a clusterIP and not necessarily reachable.

 dnsPolicy: "None"
  dnsConfig:
    nameservers:
     - 1.1.1.1
     - 1.0.0.1

EDIT3: Loop devices are not namespaced, follow from #1248 to find our current workaround