Improve error when docker-env is used with non-docker runtime

[tstromberg]

Fixes #7111

Old UI

💣  Docker runtime check failed: sudo systemctl is-active docker failed: Process exited with status 3
output: "inactive"

😿  minikube is exiting due to an error. If the above message is not useful, open an issue:
👉  https://github.com/kubernetes/minikube/issues/new/choose

New UI

💣 The docker-env command is only compatible with the "docker" runtime, but this cluster was configured to use the "cri-o" runtime.

[tstromberg]

/ok-to-test

[k8s-ci-robot]

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[minikube-pr-bot]

Error: running mkcmp: exit status 1

[afbjorklund]

Looks like this got broken with the cruntime introduction ?

Old check:

func GetDockerActive(host *host.Host) (bool, error) {
       statusCmd := `sudo systemctl is-active docker`
       status, err := host.RunSSHCommand(statusCmd)
       // systemd returns error code on inactive
       s := strings.TrimSpace(status)
       return err == nil && s == "active", err
}

New check:

// Active returns if docker is active on the host
func (r *Docker) Active() bool {
       err := r.Runner.Run("systemctl is-active --quiet service docker")
       return err == nil
}

The systemd error codes are somewhat flawed.