Custom UID and GID for subprocesses and files written (#116) Fix #116

- Environment variables `UID` and `GID`, both defaulting to `1000`
- All subprocesses (openvpn, tinyproxy, etc.) run using the UID and GID given
- All files are written with an ownership for the UID and GID given
- Port forwarded file has also ownership for UID, GID and read permission only

internal/alpine/users.go

@@ -0,0 +1,38 @@
+package alpine
+
+import (
+	"fmt"
+	"os/user"
+)
+
+// CreateUser creates a user in Alpine with the given UID
+func (c *configurator) CreateUser(username string, uid int) error {
+	UIDStr := fmt.Sprintf("%d", uid)
+	u, err := c.lookupUID(UIDStr)
+	_, unknownUID := err.(user.UnknownUserIdError)
+	if err != nil && !unknownUID {
+		return fmt.Errorf("cannot create user: %w", err)
+	} else if u != nil {
+		if u.Username == username {
+			return nil
+		}
+		return fmt.Errorf("user with ID %d exists with username %q instead of %q", uid, u.Username, username)
+	}
+	u, err = c.lookupUser(username)
+	_, unknownUsername := err.(user.UnknownUserError)
+	if err != nil && !unknownUsername {
+		return fmt.Errorf("cannot create user: %w", err)
+	} else if u != nil {
+		return fmt.Errorf("cannot create user: user with name %s already exists for ID %s instead of %d", username, u.Uid, uid)
+	}
+	passwd, err := c.fileManager.ReadFile("/etc/passwd")
+	if err != nil {
+		return fmt.Errorf("cannot create user: %w", err)
+	}
+	passwd = append(passwd, []byte(fmt.Sprintf("%s:x:%d:::/dev/null:/sbin/nologin\n", username, uid))...)
+
+	if err := c.fileManager.WriteToFile("/etc/passwd", passwd); err != nil {
+		return fmt.Errorf("cannot create user: %w", err)
+	}
+	return nil
+}