Move UserLookup functionality into a separate pkg/user submodule that implements proper parsing of /etc/passwd and /etc/group, and use that to add support for "docker run -u user:group" and for getting supplementary groups (if ":group" is not specified)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)

utils/utils.go

@@ -836,37 +836,6 @@ func ParseRepositoryTag(repos string) (string, string) {
 	return repos, ""
 }
 
-type User struct {
-	Uid      string // user id
-	Gid      string // primary group id
-	Username string
-	Name     string
-	HomeDir  string
-}
-
-// UserLookup check if the given username or uid is present in /etc/passwd
-// and returns the user struct.
-// If the username is not found, an error is returned.
-func UserLookup(uid string) (*User, error) {
-	file, err := ioutil.ReadFile("/etc/passwd")
-	if err != nil {
-		return nil, err
-	}
-	for _, line := range strings.Split(string(file), "\n") {
-		data := strings.Split(line, ":")
-		if len(data) > 5 && (data[0] == uid || data[2] == uid) {
-			return &User{
-				Uid:      data[2],
-				Gid:      data[3],
-				Username: data[0],
-				Name:     data[4],
-				HomeDir:  data[5],
-			}, nil
-		}
-	}
-	return nil, fmt.Errorf("User not found in /etc/passwd")
-}
-
 // An StatusError reports an unsuccessful exit by a command.
 type StatusError struct {
 	Status     string