Fix pull on ContainerStart wrapper used in tests (elastic#15511)

ContainerStart wrapper used in tests was not waiting for the pull to
finish, causing flakiness, e.g. in TestDockerStart. Pull can be
waited to finish by calling to Close() in the response body. This is
also something that has to be done at some moment to avoid leaks.

Also add some more context on error messages.

libbeat/tests/docker/docker.go

@@ -20,6 +20,8 @@ package docker
 import (
 	"context"
 
+	"github.com/pkg/errors"
+
 	"github.com/docker/docker/api/types"
 	"github.com/docker/docker/api/types/container"
 	"github.com/docker/docker/client"
@@ -41,21 +43,23 @@ func NewClient() (Client, error) {
 // ContainerStart pulls and starts the given container
 func (c Client) ContainerStart(image string, cmd []string, labels map[string]string) (string, error) {
 	ctx := context.Background()
-	if _, err := c.cli.ImagePull(ctx, image, types.ImagePullOptions{}); err != nil {
-		return "", err
+	respBody, err := c.cli.ImagePull(ctx, image, types.ImagePullOptions{})
+	if err != nil {
+		return "", errors.Wrapf(err, "pullling image %s", image)
 	}
+	defer respBody.Close()
 
 	resp, err := c.cli.ContainerCreate(ctx, &container.Config{
 		Image:  image,
 		Cmd:    cmd,
 		Labels: labels,
 	}, nil, nil, "")
 	if err != nil {
-		return "", err
+		return "", errors.Wrap(err, "creating container")
 	}
 
 	if err := c.cli.ContainerStart(ctx, resp.ID, types.ContainerStartOptions{}); err != nil {
-		return "", err
+		return "", errors.Wrap(err, "starting container")
 	}
 
 	return resp.ID, nil