feat(transprot): add flag for avif (#441)

transport.go

@@ -29,6 +29,7 @@ import (
 	"net"
 	"net/http"
 	"net/url"
+	"os"
 	"path"
 	"strconv"
 	"strings"
@@ -72,12 +73,13 @@ func (t *Transport) makeRequest(r *http.Request) (*http.Request, error) {
 }
 
 func scanAcceptHeader(r *http.Request) string {
+	f := os.Getenv("MANAEL_ENABLE_AVIF")
 	accepts := r.Header.Get("Accept")
 
 	for _, v := range strings.Split(accepts, ",") {
 		t := strings.TrimSpace(v)
 
-		if strings.HasPrefix(t, "image/avif") {
+		if f == "true" && strings.HasPrefix(t, "image/avif") {
 			return "image/avif"
 		} else if strings.HasPrefix(t, "image/webp") {
 			return "image/webp"