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/publicip.go

@@ -0,0 +1,51 @@
+package env
+
+import (
+	"fmt"
+	"os"
+	"time"
+
+	"github.com/qdm12/gluetun/internal/configuration/settings"
+)
+
+func (r *Reader) readPublicIP() (publicIP settings.PublicIP, err error) {
+	publicIP.Period, err = readPublicIPPeriod()
+	if err != nil {
+		return publicIP, err
+	}
+
+	publicIP.IPFilepath = r.readPublicIPFilepath()
+
+	return publicIP, nil
+}
+
+func readPublicIPPeriod() (period *time.Duration, err error) {
+	s := os.Getenv("PUBLICIP_PERIOD")
+	if s == "" {
+		return nil, nil //nolint:nilnil
+	}
+
+	period = new(time.Duration)
+	*period, err = time.ParseDuration(s)
+	if err != nil {
+		return nil, fmt.Errorf("environment variable PUBLICIP_PERIOD: %w", err)
+	}
+
+	return period, nil
+}
+
+func (r *Reader) readPublicIPFilepath() (filepath *string) {
+	s := os.Getenv("PUBLICIP_FILE")
+	if s != "" {
+		return &s
+	}
+
+	// Retro-compatibility
+	s = os.Getenv("IP_STATUS_FILE")
+	if s != "" {
+		r.onRetroActive("IP_STATUS_FILE", "PUBLICIP_FILE")
+		return &s
+	}
+
+	return nil
+}