chore(env): DNS_ADDRESS variable

- With retro-compatibility with `DNS_PLAINTEXT_ADDRESS`

internal/configuration/sources/env/dns.go

@@ -3,7 +3,6 @@ package env
 import (
 	"fmt"
 	"net"
-	"os"
 
 	"github.com/qdm12/gluetun/internal/configuration/settings"
 )
@@ -28,19 +27,19 @@ func (r *Reader) readDNS() (dns settings.DNS, err error) {
 }
 
 func (r *Reader) readDNSServerAddress() (address net.IP, err error) {
-	s := os.Getenv("DNS_PLAINTEXT_ADDRESS")
+	key, s := r.getEnvWithRetro("DNS_ADDRESS", "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)
+		return nil, fmt.Errorf("environment variable %s: %w: %s", key, 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 +
+		r.warner.Warn(key + " 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" +