Add warning for --dns flag set to localhost addresses.

We should warn users who use the `--dns` command line option to point
DNS to a localhost address, either IPv4 or IPv6.  Unless they have
specifically set up the container as a DNS server or are using
--net=host (which is why this should be allowed, but warned on because
those are pretty unique cases) a localhost address as a resolver will
not reach what they might expect (e.g. expecting it will hit localhost
on the Docker daemon/host).

Added a test for the message, and fixed up tests to separate stdout and
stderr that were using `--dns=127.0.0.1` to test the options.

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)

api/client/commands.go

@@ -37,6 +37,7 @@ import (
 	"github.com/docker/docker/pkg/fileutils"
 	"github.com/docker/docker/pkg/homedir"
 	flag "github.com/docker/docker/pkg/mflag"
+	"github.com/docker/docker/pkg/networkfs/resolvconf"
 	"github.com/docker/docker/pkg/parsers"
 	"github.com/docker/docker/pkg/parsers/filters"
 	"github.com/docker/docker/pkg/promise"
@@ -2263,6 +2264,18 @@ func (cli *DockerCli) CmdRun(args ...string) error {
 	if err != nil {
 		utils.ReportError(cmd, err.Error(), true)
 	}
+
+	if len(hostConfig.Dns) > 0 {
+		// check the DNS settings passed via --dns against
+		// localhost regexp to warn if they are trying to
+		// set a DNS to a localhost address
+		for _, dnsIP := range hostConfig.Dns {
+			if resolvconf.IsLocalhost(dnsIP) {
+				fmt.Fprintf(cli.err, "WARNING: Localhost DNS setting (--dns=%s) may fail in containers.\n", dnsIP)
+				break
+			}
+		}
+	}
 	if config.Image == "" {
 		cmd.Usage()
 		return nil