I know that pipelines is doing some caching of docker images but it doesn't seem to be reliable. For example i ran our build once and it took a couple of minutes to download our image, i ran it a second time a few minutes later and it took 25 minutes to download the image. This is a major bottleneck especially considering the fact that there are a limited number of build minutes.
This is similar to
#12818, but that is more focused on saving dependencies after the fact, whereas the bottleneck i'm describing is in downloading the docker image itself.