Dockerfiles are designed to give you all the primitives you need to compose arbitrarily complex build processes, and no more.

A Dockerfile is not a replacement for your favorite build script: it's a reliable foundation for defining, unambiguously, in which context to run your script. The Dockerfile's defining feature is that it has no implicit dependency: it only needs a working Docker install. Unlike your favorite build script, which may require "python" (but which version exactly?), ssh (but which build exactly?), gcc ( but...), openssl ( but...) and so on.