Fix loadtest/infra docker_image resource (#42537)

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**Related issue:** Resolves # N/A

- Resolves an issue that prevents some locally pulled docker images from
being pushed to ECR.

infrastructure/loadtesting/terraform/infra/docker.tf

@@ -28,6 +28,7 @@ resource "aws_ecr_repository" "fleet" {
 
 resource "docker_image" "dockerhub" {
   name = "fleetdm/fleet:${var.tag}"
+  pull_triggers = [data.docker_registry_image.dockerhub.sha256_digest]
 }
 
 data "docker_registry_image" "dockerhub" {
@@ -42,4 +43,4 @@ resource "docker_tag" "fleet" {
 resource "docker_registry_image" "fleet" {
   name          = docker_tag.fleet.target_image
   keep_remotely = true
-}
+}