The problem I see with the entire Docker ecosystem is that the Docker/Dockerfile build system is fundamentally terrible, principally because the cache system assumes a linear dependency tree and build graphs are, well, graphs. To get reasonable build performance, you need a tool that understands this. This is something Nix gets right, and it even lets you build Docker images without the Docker/Dockerfile build system.
I'd also like to see something that takes it a step further--build the docker image AND a set of kubernetes manifests that reference that docker image (a "kubernetes app package" if you will) such that you have a single artifact that represents your application that a "kubernetes package manager" can deploy (the current crop of k8s package managers altogether punt on the contents of docker images).
I understand that this is a very different way of thinking about Kubernetes and Docker builds than what we practice today, but I'd really like to see this in practice or hear some debate about its merits (or lackthereof).