Docker containers are not reproducible and building the containers takes bloody forever/poor cacheability.

Look at every Dockerfile

FROM Ubuntu/Ubuntu

RUN apt-get update (or apt-get install) # BOOM — no longer possible to reproduce the build

Sure, if you always rebuild your container images - but surely a more typical approach is to build them and put them in a container registry?

Of course, they would be rebuilt when base layers change, but if you really want exactly the same image, you reference it by a digest, which will give you a point-in-time image.