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/system.go

@@ -0,0 +1,63 @@
+package env
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"strconv"
+
+	"github.com/qdm12/gluetun/internal/configuration/settings"
+)
+
+var (
+	ErrSystemPUIDNotValid     = errors.New("PUID is not valid")
+	ErrSystemPGIDNotValid     = errors.New("PGID is not valid")
+	ErrSystemTimezoneNotValid = errors.New("timezone is not valid")
+)
+
+func (r *Reader) readSystem() (system settings.System, err error) {
+	system.PUID, err = r.readID("PUID", "UID")
+	if err != nil {
+		return system, err
+	}
+
+	system.PGID, err = r.readID("PGID", "GID")
+	if err != nil {
+		return system, err
+	}
+
+	system.Timezone = os.Getenv("TZ")
+
+	return system, nil
+}
+
+var ErrSystemIDNotValid = errors.New("system ID is not valid")
+
+func (r *Reader) readID(key, retroKey string) (
+	id *uint16, err error) {
+	idEnvKey := key
+	idString := os.Getenv(key)
+	if idString == "" {
+		// retro-compatibility
+		idString = os.Getenv(retroKey)
+		if idString != "" {
+			idEnvKey = retroKey
+			r.onRetroActive(retroKey, key)
+		}
+	}
+
+	if idString == "" {
+		return nil, nil //nolint:nilnil
+	}
+
+	idInt, err := strconv.Atoi(idString)
+	if err != nil {
+		return nil, fmt.Errorf("environment variable %s: %w: %s: %s",
+			idEnvKey, ErrSystemIDNotValid, idString, err)
+	} else if idInt < 0 || idInt > 2^16 {
+		return nil, fmt.Errorf("environment variable %s: %w: %d: must be between 0 and 65535",
+			idEnvKey, ErrSystemIDNotValid, idInt)
+	}
+
+	return uint16Ptr(uint16(idInt)), nil
+}