Adds check if default ip address is correct format.

It avoids hidden error when ports are redirected from
container to host using -p host_port:guest_port.

Docker-DCO-1.1-Signed-off-by: Jiří Župka <jzupka@redhat.com> (github: jzupka)

docker/docker.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"io/ioutil"
 	"log"
+	"net"
 	"os"
 	"runtime"
 	"strings"
@@ -99,6 +100,10 @@ func main() {
 		log.Fatal("You specified --iptables=false with --icc=false. ICC uses iptables to function. Please set --icc or --iptables to true.")
 	}
 
+	if net.ParseIP(*flDefaultIp) == nil {
+		log.Fatalf("Specified --ip=%s is not in correct format \"0.0.0.0\".", *flDefaultIp)
+	}
+
 	if *flDebug {
 		os.Setenv("DEBUG", "1")
 	}