systemtest: don't pull elastic-agent image (#7162) (#7174)

I think at some point in the past there was a separate
image for fleet-server, and we separately pulled the
elastic-agent image. Fleet-server now (at least) runs
under elastic-agent, so we can use the exact same image
and avoid a network trip.

(cherry picked from commit 8010904)

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

systemtest/containers.go

@@ -325,12 +325,9 @@ func NewUnstartedElasticAgentContainer() (*ElasticAgentContainer, error) {
 	systemtestDir := filepath.Dir(filename)
 	hostCACertPath := filepath.Join(systemtestDir, "../testing/docker/fleet-server/ca.pem")
 
-	// Use the same stack version as used for fleet-server.
+	// Use the same elastic-agent image as used for fleet-server.
 	agentImageVersion := fleetServerContainer.Image[strings.LastIndex(fleetServerContainer.Image, ":")+1:]
 	agentImage := "docker.elastic.co/beats/elastic-agent:" + agentImageVersion
-	if err := pullDockerImage(context.Background(), docker, agentImage); err != nil {
-		return nil, err
-	}
 	agentImageDetails, _, err := docker.ImageInspectWithRaw(context.Background(), agentImage)
 	if err != nil {
 		return nil, err
@@ -356,10 +353,9 @@ func NewUnstartedElasticAgentContainer() (*ElasticAgentContainer, error) {
 		SkipReaper: true, // we use our own reaping logic
 	}
 	return &ElasticAgentContainer{
-		request:      req,
-		exited:       make(chan struct{}),
-		Reap:         true,
-		StackVersion: agentImageVersion,
+		request: req,
+		exited:  make(chan struct{}),
+		Reap:    true,
 	}, nil
 }
 
@@ -375,10 +371,6 @@ type ElasticAgentContainer struct {
 	// from being stoped and removed.
 	Reap bool
 
-	// StackVersion holds the stack version of the container image,
-	// e.g. 8.0.0-SNAPSHOT.
-	StackVersion string
-
 	// ExposedPorts holds an optional list of ports to expose to the host.
 	ExposedPorts []string
 
@@ -570,16 +562,6 @@ func (c *ElasticAgentContainer) Exec(ctx context.Context, cmd ...string) (stdout
 	return stdoutBuf.Bytes(), stderrBuf.Bytes(), nil
 }
 
-func pullDockerImage(ctx context.Context, docker *client.Client, imageRef string) error {
-	rc, err := docker.ImagePull(context.Background(), imageRef, types.ImagePullOptions{})
-	if err != nil {
-		return err
-	}
-	defer rc.Close()
-	_, err = io.Copy(ioutil.Discard, rc)
-	return err
-}
-
 func matchFleetServerAPIStatusHealthy(r io.Reader) bool {
 	var status struct {
 		Name    string `json:"name"`