GTC-2958 Fix reference to non-existent ECR image

The container_registry terraform module has the functionality that it
will create a new docker image with the specified tag, but only if the
docker contents have changed (as computed by its hash script, which
understands .dockerignore, etc.)

We use as a container tag the git SHA, which is always different when we
deploy a change. But if only terraform is being changed, no new docker
image will be created, so there will be no image with that tag, leading
to the bug.

Since the ECR registries are separated by GFW account, and within
GFW-dev by the terraform.workspace name (branch), I believe we can
instead change our tag reference in data.tf:template_file.container_definition
to "latest" (which always exists), rather than the GIT sha tag, which
may not exist.

Other possible solutions:
 - always create a new docker image for every GIT change
 - make the container tag be the MD5 of the docker contents, which would mean
   replicating the hash script in gfw-data-api, so we can compute it ahead of
   time.  Then we would always refer to a tag of an image that already exists
   or was just created.

terraform/data.tf

@@ -37,7 +37,7 @@ data "terraform_remote_state" "tile_cache" {
 data "template_file" "container_definition" {
   template = file("${path.root}/templates/container_definition.json.tmpl")
   vars = {
-    image = "${module.app_docker_image.repository_url}:${local.container_tag}"
+    image = "${module.app_docker_image.repository_url}:latest"
 
     container_name = var.container_name
     container_port = var.container_port
@@ -190,4 +190,4 @@ data "template_file" "step_function_policy" {
   vars = {
     raster_analysis_state_machine_arn = data.terraform_remote_state.raster_analysis_lambda.outputs.raster_analysis_state_machine_arn
   }
-}
+}