Add username to /etc/passwd inside of container if --userns keep-id

If I enter a continer with --userns keep-id, my UID will be present
inside of the container, but most likely my user will not be defined.

This patch will take information about the user and stick it into the
container.

Signed-off-by: Daniel J Walsh <dwalsh@redhat.com>

libpod/container_internal_linux_test.go

@@ -0,0 +1,42 @@
+// +build linux
+
+package libpod
+
+import (
+	"io/ioutil"
+	"os"
+	"testing"
+
+	spec "github.com/opencontainers/runtime-spec/specs-go"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGenerateUserPasswdEntry(t *testing.T) {
+	dir, err := ioutil.TempDir("", "libpod_test_")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+
+	c := Container{
+		config: &ContainerConfig{
+			User: "123:456",
+			Spec: &spec.Spec{},
+		},
+		state: &ContainerState{
+			Mountpoint: "/does/not/exist/tmp/",
+		},
+	}
+	user, err := c.generateUserPasswdEntry()
+	if err != nil {
+		t.Fatal(err)
+	}
+	assert.Equal(t, user, "123:x:123:456:container user:/:/bin/sh\n")
+
+	c.config.User = "567"
+	user, err = c.generateUserPasswdEntry()
+	if err != nil {
+		t.Fatal(err)
+	}
+	assert.Equal(t, user, "567:x:567:0:container user:/:/bin/sh\n")
+}