Hello,

In a CI environment where no layer cache are available, docker builds of ECS tasks can take a very long time.

A classic solution for this is to pull first the latest image from the registry and use it as an input for the --cache-from argument of docker build (see here aws/aws-codebuild-docker-images#26 (comment) and https://medium.com/@gajus/making-docker-in-docker-builds-x2-faster-using-docker-cache-from-option-c01febd8ef84).

This implies the following changes in prepareContainerAsset:

• In addition to tagging the image with the calculateImageFingerprint, tag with latest
• When in CI (detected by something like process.env.CI) try to pull latest first and then build with --cache-from

(toolkitInfo.prepareEcrRepository should also be refactored/splitted in order to get the repostoryUri sooner/without having to pass a tag)

I would be interested to have aws-cdk's team opinion on this.