Add 504 to the list of responses we retry on

This is "gateway timeout" which we do see from Docker Hub from time to time (probably due to some HTTP middleware, redeploys, etc).

registry/rate-limits.go

@@ -2,6 +2,7 @@ package registry
 
 import (
 	"net/http"
+	"slices"
 	"time"
 
 	"golang.org/x/time/rate"
@@ -24,7 +25,7 @@ func (d *rateLimitedRetryingRoundTripper) RoundTrip(req *http.Request) (*http.Re
 		// cap request retries at once per second
 		requestRetryLimiter = rate.NewLimiter(rate.Every(time.Second), 1)
 
-		// if we see 3x (503 or 502 or 500) during retry, we should bail
+		// if we see 50x three times during retry, we should bail
 		maxTry50X = 3
 
 		ctx = req.Context()
@@ -54,7 +55,7 @@ func (d *rateLimitedRetryingRoundTripper) RoundTrip(req *http.Request) (*http.Re
 		}
 
 		// certain status codes should result in a few auto-retries (especially with the automatic retry delay this injects), but up to a limit so we don't contribute to the "thundering herd" too much in a serious outage
-		if (res.StatusCode == 503 || res.StatusCode == 502 || res.StatusCode == 500) && maxTry50X > 1 {
+		if maxTry50X > 1 && slices.Contains([]int{500, 502, 503, 504}, res.StatusCode) {
 			maxTry50X--
 			doRetry = true
 			// no need to eat up the rate limiter tokens as we do for 429 because this is not a rate limiting error (and we have the "requestRetryLimiter" that separately limits our retries of *this* request)