chore(settings): refactor settings processing (#756)

- Better settings tree structure logged using `qdm12/gotree`
- Read settings from environment variables, then files, then secret files
- Settings methods to default them, merge them and override them
- `DNS_PLAINTEXT_ADDRESS` default changed to `127.0.0.1` to use DoT. Warning added if set to something else.
- `HTTPPROXY_LISTENING_ADDRESS` instead of `HTTPPROXY_PORT` (with retro-compatibility)

internal/configuration/sources/env/log.go

@@ -0,0 +1,54 @@
+package env
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"strings"
+
+	"github.com/qdm12/gluetun/internal/configuration/settings"
+	"github.com/qdm12/golibs/logging"
+)
+
+func readLog() (log settings.Log, err error) {
+	log.Level, err = readLogLevel()
+	if err != nil {
+		return log, err
+	}
+
+	return log, nil
+}
+
+func readLogLevel() (level *logging.Level, err error) {
+	s := os.Getenv("LOG_LEVEL")
+	if s == "" {
+		return nil, nil //nolint:nilnil
+	}
+
+	level = new(logging.Level)
+	*level, err = parseLogLevel(s)
+	if err != nil {
+		return nil, fmt.Errorf("environment variable LOG_LEVEL: %w", err)
+	}
+
+	return level, nil
+}
+
+var ErrLogLevelUnknown = errors.New("log level is unknown")
+
+func parseLogLevel(s string) (level logging.Level, err error) {
+	switch strings.ToLower(s) {
+	case "debug":
+		return logging.LevelDebug, nil
+	case "info":
+		return logging.LevelInfo, nil
+	case "warning":
+		return logging.LevelWarn, nil
+	case "error":
+		return logging.LevelError, nil
+	default:
+		return level, fmt.Errorf(
+			"%w: %s: can be one of: debug, info, warning or error",
+			ErrLogLevelUnknown, s)
+	}
+}