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/dns.go

@@ -0,0 +1,51 @@
+package env
+
+import (
+	"fmt"
+	"net"
+	"os"
+
+	"github.com/qdm12/gluetun/internal/configuration/settings"
+)
+
+func (r *Reader) readDNS() (dns settings.DNS, err error) {
+	dns.ServerAddress, err = r.readDNSServerAddress()
+	if err != nil {
+		return dns, err
+	}
+
+	dns.KeepNameserver, err = envToBoolPtr("DNS_KEEP_NAMESERVER")
+	if err != nil {
+		return dns, fmt.Errorf("environment variable DNS_KEEP_NAMESERVER: %w", err)
+	}
+
+	dns.DoT, err = r.readDoT()
+	if err != nil {
+		return dns, fmt.Errorf("cannot read DoT settings: %w", err)
+	}
+
+	return dns, nil
+}
+
+func (r *Reader) readDNSServerAddress() (address net.IP, err error) {
+	s := os.Getenv("DNS_PLAINTEXT_ADDRESS")
+	if s == "" {
+		return nil, nil
+	}
+
+	address = net.ParseIP(s)
+	if address == nil {
+		return nil, fmt.Errorf("environment variable DNS_PLAINTEXT_ADDRESS: %w: %s", ErrIPAddressParse, s)
+	}
+
+	// TODO remove in v4
+	if !address.Equal(net.IPv4(127, 0, 0, 1)) { //nolint:gomnd
+		r.warner.Warn("DNS_PLAINTEXT_ADDRESS is set to " + s +
+			" so the DNS over TLS (DoT) server will not be used." +
+			" The default value changed to 127.0.0.1 so it uses the internal DoT server." +
+			" If the DoT server fails to start, the IPv4 address of the first plaintext DNS server" +
+			" corresponding to the first DoT provider chosen is used.")
+	}
+
+	return address, nil
+}