moby: api.go doesn't respect nsswitch.conf

Due to the implementation of api.go which reads only /etc/group, it is impossible to rely on other auth mechanisms (e.g. LDAP, NIS).

groups, err := ioutil.ReadFile("/etc/group")
if err != nil {
        return err
}
re := regexp.MustCompile("(^|\n)docker:.*?:([0-9]+)")
if gidMatch := re.FindStringSubmatch(string(groups)); gidMatch != nil {
        gid, err := strconv.Atoi(gidMatch[2])
        if err != nil {
                return err
        }
        utils.Debugf("docker group found. gid: %d", gid)
        if err := os.Chown(addr, 0, gid); err != nil {
                return err
        }
}

Instead, the getgrnam syscall should be used to determine, whether a docker group exists or not.

The implemenation of lookup_unix.go in the user Go package might serve as inspiration on how to do this.