Clarify Terraform bootstrap image tag ownership (#4)

- Add `bootstrap_image_tag` Terraform variable for the initial ECS task definition image tag
- Replace hardcoded `:latest` in Terraform task definition with `var.bootstrap_image_tag`
- Document the ownership split between Terraform bootstrap infrastructure and GitHub Actions SHA-based CD
- Clarify that Terraform should not roll back externally deployed ECS task definition revisions

Verified:
- `terraform fmt`
- `terraform validate`
- `terraform plan`
- Plan reported no infrastructure changes because `bootstrap_image_tag` defaults to `latest`

Author: keithataylor

docs/aws_terraform_deployment_sequence.md

@@ -752,7 +752,6 @@ Deferred production hardening:
 - production credential registry/admin process
 - migration version table
 - CI-before-deploy safety clarification and deployment guardrails
-- Terraform image tag handling alignment with SHA-based CD
 
 ---
 
@@ -804,7 +803,42 @@ Important distinction:
 
 ---
 
-## 20. Implemented AWS runtime mapping
+## 20. Clarified Terraform bootstrap image tag ownership
+
+Files:
+
+- `infra/terraform/ecs_task_definition.tf`
+- `infra/terraform/variables.tf`
+- `infra/terraform/ecs_service.tf`
+
+Why:
+
+- Terraform needs an image reference so it can create the initial ECS task definition.
+- GitHub Actions CD owns real application releases after infrastructure exists.
+- The CD workflow deploys immutable Git commit SHA image tags by registering new ECS task definition revisions.
+- Terraform should not roll back the ECS service to its original bootstrap task definition during later infrastructure applies.
+
+Implemented changes:
+
+- Added `var.bootstrap_image_tag`, defaulting to `latest`.
+- Updated the Terraform ECS task definition image reference to use `var.bootstrap_image_tag`.
+- Kept the ECS service lifecycle rule that ignores `task_definition` and `desired_count`.
+
+Important distinction:
+
+- Terraform owns the ECS task definition shape and bootstrap image reference.
+- GitHub Actions CD owns the currently deployed application image revision.
+- Manual operations may temporarily own `desired_count`, for example when pausing the service at zero.
+- Terraform should not treat SHA-based CD deployments as infrastructure drift.
+
+Verified result:
+
+- `terraform fmt` completed.
+- `terraform validate` passed.
+- `terraform plan` reported no changes because `var.bootstrap_image_tag` still defaults to `latest`.
+
+
+## 21. Implemented AWS runtime mapping
 
 Local Docker Compose mapping:
 
@@ -838,7 +872,7 @@ The application settings code continues reading the same variable names. The dep
 
 ---
 
-## 21. Local-to-AWS environment mapping
+## 22. Local-to-AWS environment mapping
 
 Local app configuration:
 
@@ -899,7 +933,7 @@ Example smoke checks:
 
 ---
 
-## 22. Why so many explicit resources are required
+## 23. Why so many explicit resources are required
 
 AWS does not infer the runtime wiring automatically.
 

infra/terraform/ecs_task_definition.tf

@@ -16,7 +16,7 @@ resource "aws_ecs_task_definition" "app_task_definition" {
   container_definitions = jsonencode([
     {
       name      = "app"
-      image     = "${aws_ecr_repository.app.repository_url}:latest"
+      image     = "${aws_ecr_repository.app.repository_url}:${var.bootstrap_image_tag}"
       essential = true
 
       portMappings = [

infra/terraform/variables.tf

@@ -19,4 +19,10 @@ variable "environment" {
 variable "agent_credential_hash_secret_arn" {
   description = "ARN of the existing Secrets Manager secret containing AGENT_CREDENTIAL_HASH_SECRET."
   type        = string
-}
+}
+
+variable "bootstrap_image_tag" {
+  description = "Initial image tag used by Terraform when creating the bootstrap ECS task definition. GitHub Actions CD deploys immutable Git SHA image tags after infrastructure exists."
+  type        = string
+  default     = "latest"
+}