Dockerfile.build: Downgrade debian to stretch

This downgrades the debian container to oldstable, and re-implements the upstream rust/cargo container at that.

The reasoning for this is to allow us build & deploy binaries on debian 9, which due to how glibc-versioning works requires us to have an older glibc to link against.

As the arm32-musl target isn't stable yet for rust, and we also need gcc-implementations, it was hairier than I felt in the mood to deal with to enable cross-compilation.

Issue: #55 (closed)