Enhance Dockerfile and documentation for GeoIP directory permissions

- Updated Dockerfile to create and set ownership for the /var/lib/telemt-gateway directory, ensuring the gateway user has the necessary permissions.
- Added documentation in GEOIP.md to clarify directory permissions required for GeoIP data downloads, including guidance on volume mounting and user ID consistency.

internal/geoip/download.go

@@ -38,11 +38,11 @@ func downloadRawMMDB(ctx context.Context, client *http.Client, url, destPath str
 	}
 	dir := filepath.Dir(destPath)
 	if err := os.MkdirAll(dir, 0o755); err != nil {
-		return fmt.Errorf("geoip mkdir: %w", err)
+		return fmt.Errorf("geoip mkdir %q: %w (directory must exist or be creatable by the process)", dir, err)
 	}
 	tmp, err := os.CreateTemp(dir, "geolite-*.mmdb.part")
 	if err != nil {
-		return err
+		return fmt.Errorf("geoip temp file in %q: %w", dir, err)
 	}
 	tmpPath := tmp.Name()
 	_, copyErr := io.Copy(tmp, resp.Body)
@@ -87,11 +87,11 @@ func downloadGzippedMMDB(ctx context.Context, client *http.Client, url, destPath
 
 	dir := filepath.Dir(destPath)
 	if err := os.MkdirAll(dir, 0o755); err != nil {
-		return fmt.Errorf("geoip mkdir: %w", err)
+		return fmt.Errorf("geoip mkdir %q: %w (directory must exist or be creatable by the process)", dir, err)
 	}
 	tmp, err := os.CreateTemp(dir, "geolite-*.mmdb.part")
 	if err != nil {
-		return err
+		return fmt.Errorf("geoip temp file in %q: %w", dir, err)
 	}
 	tmpPath := tmp.Name()
 	_, copyErr := io.Copy(tmp, gzr)