fix(cache): build job URLs on the address its runner registered (#1153)

The cache server built every URL it hands a job from its own listen address, so jobs whose runner reaches it through a reverse proxy were sent to the internal one. This covered the v1 `archiveLocation`, the v2 signed cache URLs and `ACTIONS_RESULTS_URL`.

Runners now register the address their jobs reach the server at, next to the instance URL they already send. The cache-server needs no configuration of its own, and runners that reach it differently each get their own correct address.

Closes https://gitea.com/gitea/runner/issues/1152

---------

Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1153
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>
Reviewed-by: bircni <bircni@icloud.com>
Co-authored-by: Max P. <mail@0xMax42.io>

internal/app/run/runner.go

@@ -605,14 +605,15 @@ func (r *Runner) registerExternalCacheJob(token string, cred artifactcache.JobCr
 	resultsURL := ""
 	if body, err := postInternalCache(base+"/_internal/register", r.cfg.Cache.ExternalSecret, map[string]any{
 		"token": token, "repo": cred.Repo, "results": cred.Results, "insecure_tls": cred.InsecureTLS,
+		"public_url": base,
 	}); err != nil {
 		log.Warnf("cache external_server register failed (%s): %v", base, err)
 		if reporter != nil {
 			reporter.Logf("::warning::%s", runner.EscapeCommandData(fmt.Sprintf(
 				"cache external_server register failed (%s): %v — cache requests from this job will be unauthenticated and likely return 401", base, err)))
 		}
-	} else {
-		resultsURL, _ = body["results_url"].(string) // absent from a server too old to forward
+	} else if forwarded, _ := body["results_url"].(string); forwarded != "" {
+		resultsURL = base // the answer only says it forwards, its own address need not be the job's
 	}
 	return func() {
 		if _, err := postInternalCache(base+"/_internal/revoke", r.cfg.Cache.ExternalSecret,