feat(storage): STORAGE_FILEPATH option (#2416)

- `STORAGE_FILEPATH=` disables storing to and reading from a local servers.json file
- `STORAGE_FILEPATH` defaults to `/gluetun/servers.json`
- Fix #2074

internal/configuration/settings/storage.go

@@ -0,0 +1,59 @@
+package settings
+
+import (
+	"fmt"
+	"path/filepath"
+
+	"github.com/qdm12/gosettings"
+	"github.com/qdm12/gosettings/reader"
+	"github.com/qdm12/gotree"
+)
+
+// Storage contains settings to configure the storage.
+type Storage struct {
+	// Filepath is the path to the servers.json file. An empty string disables on-disk storage.
+	Filepath *string
+}
+
+func (s Storage) validate() (err error) {
+	if *s.Filepath != "" { // optional
+		_, err := filepath.Abs(*s.Filepath)
+		if err != nil {
+			return fmt.Errorf("filepath is not valid: %w", err)
+		}
+	}
+	return nil
+}
+
+func (s *Storage) copy() (copied Storage) {
+	return Storage{
+		Filepath: gosettings.CopyPointer(s.Filepath),
+	}
+}
+
+func (s *Storage) overrideWith(other Storage) {
+	s.Filepath = gosettings.OverrideWithPointer(s.Filepath, other.Filepath)
+}
+
+func (s *Storage) setDefaults() {
+	const defaultFilepath = "/gluetun/servers.json"
+	s.Filepath = gosettings.DefaultPointer(s.Filepath, defaultFilepath)
+}
+
+func (s Storage) String() string {
+	return s.toLinesNode().String()
+}
+
+func (s Storage) toLinesNode() (node *gotree.Node) {
+	if *s.Filepath == "" {
+		return gotree.New("Storage settings: disabled")
+	}
+	node = gotree.New("Storage settings:")
+	node.Appendf("Filepath: %s", *s.Filepath)
+	return node
+}
+
+func (s *Storage) read(r *reader.Reader) (err error) {
+	s.Filepath = r.Get("STORAGE_FILEPATH", reader.AcceptEmpty(true))
+	return nil
+}