Don't block server boot if there is an error pulling an image

Obviously this requires the image to exist locally, but should avoid widespread issues when Quay inevitably goes down again.

closes pterodactyl/panel#2076

server/environment_docker.go

@@ -547,12 +547,45 @@ func (d *DockerEnvironment) DisableResourcePolling() error {
 	return errors.WithStack(err)
 }
 
-// Pulls the image from Docker.
+// Pulls the image from Docker. If there is an error while pulling the image from the source
+// but the image already exists locally, we will report that error to the logger but continue
+// with the process.
+//
+// The reasoning behind this is that Quay has had some serious outages as of late, and we don't
+// need to block all of the servers from booting just because of that. I'd imagine in a lot of
+// cases an outage shouldn't affect users too badly. It'll at least keep existing servers working
+// correctly if anything.
 //
 // @todo handle authorization & local images
 func (d *DockerEnvironment) ensureImageExists(c *client.Client) error {
-	out, err := c.ImagePull(context.Background(), d.Server.Container.Image, types.ImagePullOptions{All: false})
+	ctx, _ := context.WithTimeout(context.Background(), time.Second*10)
+
+	out, err := c.ImagePull(ctx, d.Server.Container.Image, types.ImagePullOptions{All: false})
 	if err != nil {
+		images, ierr := c.ImageList(ctx, types.ImageListOptions{})
+		if ierr != nil {
+			// Well damn, something has gone really wrong here, just go ahead and abort there
+			// isn't much anything we can do to try and self-recover from this.
+			return ierr
+		}
+
+		for _, img := range images {
+			for _, t := range img.RepoTags {
+				if t == d.Server.Container.Image {
+					zap.S().Warnw(
+						"unable to pull requested image from remote source, however the image exists locally",
+						zap.String("server", d.Server.Uuid),
+						zap.String("image", d.Server.Container.Image),
+						zap.Error(err),
+					)
+
+					// Okay, we found a matching container image, in that case just go ahead and return
+					// from this function, since there is nothing else we need to do here.
+					return nil
+				}
+			}
+		}
+
 		return err
 	}
 	defer out.Close()