systemtest: don't use fleet-server IP (#7157) (#7158)

I don't recall why we were using the IP for fleet-server
in the first place, but should be able to rely on the
container's hostname being resolvable, and it allows us
to verify TLS without relying on IP SANs.

(cherry picked from commit d57b999)

Co-authored-by: Andrew Wilkins <[email protected]>

systemtest/containers.go

@@ -54,8 +54,6 @@ import (
 
 const (
 	startContainersTimeout = 5 * time.Minute
-
-	fleetServerPort = "8220"
 )
 
 var (
@@ -314,17 +312,9 @@ func NewUnstartedElasticAgentContainer() (*ElasticAgentContainer, error) {
 		return nil, err
 	}
 
-	var fleetServerIPAddress string
 	var networks []string
-	for network, settings := range fleetServerContainerDetails.NetworkSettings.Networks {
+	for network := range fleetServerContainerDetails.NetworkSettings.Networks {
 		networks = append(networks, network)
-		if fleetServerIPAddress == "" && settings.IPAddress != "" {
-			fleetServerIPAddress = settings.IPAddress
-		}
-	}
-	fleetServerURL := &url.URL{
-		Scheme: "https",
-		Host:   net.JoinHostPort(fleetServerIPAddress, fleetServerPort),
 	}
 	containerCACertPath := "/etc/pki/tls/certs/fleet-ca.pem"
 
@@ -360,7 +350,7 @@ func NewUnstartedElasticAgentContainer() (*ElasticAgentContainer, error) {
 		Networks:   networks,
 		BindMounts: map[string]string{hostCACertPath: containerCACertPath},
 		Env: map[string]string{
-			"FLEET_URL": fleetServerURL.String(),
+			"FLEET_URL": "https://fleet-server:8220",
 			"FLEET_CA":  containerCACertPath,
 		},
 		SkipReaper: true, // we use our own reaping logic