Switch to pytorch-style CI docker image naming

[jithunnair-amd]

PyTorch recently switched to having all the config information in the CI docker image tag instead of the repo: pytorch/pytorch#152209

Change CI docker images to be ci-image:- instead of : so we never have to make a new ecr repo ever again

Torchtitan should switch to the same model, so that it's frictionless to create new CI docker images or update the docker image names in case the ROCm/OS/clang etc. version changes: #1260 (comment)

cc @huydhn @akashveramd @tianyu-l

[huydhn]

Here is an example PR on how to do this from @clee2000 pytorch/executorch#12599 (review)

[wwwjn]

@tianyu-l I could take this one as I'm refactoring CI recently