So I can give some behind the scenes insight on that. I don't think image caching will be a thing in the way people are explicitly asking, but we are exploring some alternative approaches to speeding up container launch that we think will actually be even more effective than what people are asking for.
First of all we want to leverage some of the learnings from AWS Lambda, in specific some of the research we've done that shows that about 75% of container images only contain 5% unique bytes (https://brooker.co.za/blog/2023/05/23/snapshot-loading.html). This makes deduplication incredibly effective, and allows the deployment of a smart cache that holds the 95% of popular recurring files and file chunks from container images, while letting the unique 5% be loaded over the network. There will be outliers of course, but if you base your image off a well used base image then it will already be in the cache. This is partially implemented. You will notice that if you use certain base images your Fargate tasks seems to start a bit faster. (Unfortunately we do not really publish this list or commit to what base images are in the cache at this time).
In another step along this path we are working on SOCI Snapshotter (https://github.com/awslabs/soci-snapshotter) forked off of Stargz Snapshotter. This allows a container image to have an attached index file that actually allows it to start up before all the contents are downloaded, and lazy load in remaining chunks of the image as needed. This takes advantage of another aspect of container images which is that many of them don't actually use all of the bytes in the image anyway.
Over time we want to make these two pieces (deduplication and lazy loading) completely behind the scenes so you just upload your image to Elastic Container Registry and AWS Fargate seems to magically start your image dramatically faster than you could locally if downloading the image from scratch.