integration: update getExternalAddress to prefer IPv4

Rootlesskit doesn't currently handle IPv6 addresses, causing TestNetworkLoopbackNat
and TestNetworkNat to fail;

    Error starting userland proxy:
    error while calling PortManager.AddPort(): listen tcp: address :::8080: too many colons in address

This patch:

- Updates `getExternalAddress()` to pick IPv4 address if both IPv6 and IPv4 are found
- Update TestNetworkNat to net.JoinHostPort(), so that square brackets are used for
  IPv6 addresses (e.g. `[::]:8080`)

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit f845b98ca6)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

integration/container/nat_test.go

@@ -30,7 +30,7 @@ func TestNetworkNat(t *testing.T) {
 	startServerContainer(t, msg, 8080)
 
 	endpoint := getExternalAddress(t)
-	conn, err := net.Dial("tcp", fmt.Sprintf("%s:%d", endpoint.String(), 8080))
+	conn, err := net.Dial("tcp", net.JoinHostPort(endpoint.String(), "8080"))
 	assert.NilError(t, err)
 	defer conn.Close()
 
@@ -115,6 +115,9 @@ func startServerContainer(t *testing.T, msg string, port int) string {
 	return cID
 }
 
+// getExternalAddress() returns the external IP-address from eth0. If eth0 has
+// multiple IP-addresses, it returns the first IPv4 IP-address; if no IPv4
+// address is present, it returns the first IP-address found.
 func getExternalAddress(t *testing.T) net.IP {
 	t.Helper()
 	iface, err := net.InterfaceByName("eth0")
@@ -124,6 +127,17 @@ func getExternalAddress(t *testing.T) net.IP {
 	assert.NilError(t, err)
 	assert.Check(t, 0 != len(ifaceAddrs))
 
+	if len(ifaceAddrs) > 1 {
+		// Prefer IPv4 address if multiple addresses found, as rootlesskit
+		// does not handle IPv6 currently https://github.com/moby/moby/pull/41908#issuecomment-774200001
+		for _, a := range ifaceAddrs {
+			ifaceIP, _, err := net.ParseCIDR(a.String())
+			assert.NilError(t, err)
+			if ifaceIP.To4() != nil {
+				return ifaceIP
+			}
+		}
+	}
 	ifaceIP, _, err := net.ParseCIDR(ifaceAddrs[0].String())
 	assert.NilError(t, err)