Expedited image manifest digest reporting

[amogh09]

Summary

This PR contains all the changes for expedited image digest reporting enhancement. The functional changes are listed below.

• For a task container that does not have a digest specified in its Image field in the task payload, Agent will resolve the manifest digest for the container image during the container's transition to MANIFEST_PULLED state. Digest will be resolved by calling the image repository if image pull is required for the container and by inspecting a locally available image if image pull is not required for the container.
• If at least one container of the task had its digest resolved during transition to MANIFEST_PULLED state, then Agent will make a STSC (SubmitTaskStateChange) call to ECS backend to report all the resolved digests for the task.
• If a container had its image digest resolved during transition to MANIFEST_PULLED state, then a canonical image reference, prepared using the image repository name and the resolved digest, will be used for pulling the container image. For example, instead of docker image pull public.ecr.aws/library/busybox:latest, Agent will perform the equivalent of docker image pull public.ecr.aws/library/busybox@sha256:<resolved-digest>. After pulling the image, Agent will tag the pulled image with the value of container's Image field so that the image is discoverable on the host using the container's Image field.

Impact

Users can notice these changes in a few ways.

• Some additional delay is expected to task start times due to the overhead of expedited digest resolution. The exact delay would depend on the image registry for the container images. During our testing with public and ECR images, and Dockerhub images with unlimited pulls allowed, we observed additional task start delay under 500ms on average.
• Users' image repositories will see one additional call to their v2 manifest endpoints per task launch.
• For each task requiring digest resolution, there will be one additional SubmitTaskStateChange API call made on customer's behalf by Agent. This call belongs to "Agent modify actions" API action category.

Approved PRs included

This PR merges a feature branch for expedited digest reporting feature to dev branch. The individual PRs included in this PR were reviewed in the past and are listed below.

• Implement transition function for MANIFEST_PULLED state #4152
• Use resolved digest for image pulls #4157
• Additional integration tests for digest resolution changes #4161
• Enable SubmitTaskStateChange for early digest reporting #4169

Testing

New unit, integration, and functional tests have been added.

Comprehensive manual testing was performed. Stress testing was performed to measure the additional task start delay (results noted above) and it was decided that the additional task start delay is acceptable.

New tests cover the changes: yes

Description for the changelog

Feature: Expedited reporting of container image manifest digests to ECS backend. This change makes Agent resolve container image manifest digests for container images prior to image pulls by either calling image registries or inspecting local images depending on the host state and Agent configuration. Resolved digests will be reported to ECS backend using an additional SubmitTaskStateChange API call.

Does this PR include breaking model changes? If so, Have you added transformation functions?

No breaking model changes included.

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.