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/unbound.go

@@ -0,0 +1,38 @@
+package env
+
+import (
+	"fmt"
+
+	"github.com/qdm12/gluetun/internal/configuration/settings"
+)
+
+func readUnbound() (unbound settings.Unbound, err error) {
+	unbound.Providers = envToCSV("DOT_PROVIDERS")
+
+	unbound.Caching, err = envToBoolPtr("DOT_CACHING")
+	if err != nil {
+		return unbound, fmt.Errorf("environment variable DOT_CACHING: %w", err)
+	}
+
+	unbound.IPv6, err = envToBoolPtr("DOT_IPV6")
+	if err != nil {
+		return unbound, fmt.Errorf("environment variable DOT_IPV6: %w", err)
+	}
+
+	unbound.VerbosityLevel, err = envToUint8Ptr("DOT_VERBOSITY")
+	if err != nil {
+		return unbound, fmt.Errorf("environment variable DOT_VERBOSITY: %w", err)
+	}
+
+	unbound.VerbosityDetailsLevel, err = envToUint8Ptr("DOT_VERBOSITY_DETAILS")
+	if err != nil {
+		return unbound, fmt.Errorf("environment variable DOT_VERBOSITY_DETAILS: %w", err)
+	}
+
+	unbound.ValidationLogLevel, err = envToUint8Ptr("DOT_VALIDATION_LOGLEVEL")
+	if err != nil {
+		return unbound, fmt.Errorf("environment variable DOT_VALIDATION_LOGLEVEL: %w", err)
+	}
+
+	return unbound, nil
+}