fix: resolve ruff PLR2004 lint error and Windows integration test failure

- Extract magic number 3 to _ECS_SERVICE_ARN_PARTS constant
- Switch test Dockerfile to public.ecr.aws alpine (multi-arch, no rate limits)

Author: singledigit

samcli/lib/sync/flows/ecs_container_sync_flow.py

@@ -21,6 +21,10 @@
 
 LOG = logging.getLogger(__name__)
 
+# Minimum number of parts expected when splitting an ECS service ARN by "/"
+# e.g. arn:aws:ecs:region:account:service/cluster/name -> [..., "cluster", "name"]
+_ECS_SERVICE_ARN_PARTS = 3
+
 
 class ECSContainerSyncFlow(SyncFlow):
     """SyncFlow for ECS TaskDefinition and AgentCore container image resources.
@@ -174,7 +178,7 @@ def _force_ecs_deployment(self) -> None:
                 try:
                     # Extract cluster and service from the ARN
                     parts = resource_physical_id.rsplit("/", 2)
-                    if len(parts) < 3:
+                    if len(parts) < _ECS_SERVICE_ARN_PARTS:
                         continue
                     cluster = parts[-2]
                     service_name = parts[-1]

tests/integration/testdata/buildcmd/container_image/agent/Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.12-slim
+FROM public.ecr.aws/docker/library/alpine:3.19
 WORKDIR /app
 RUN echo "hello" > /app/test.txt
-CMD ["python", "-c", "print('test')"]
+CMD ["echo", "test"]