releases: add Docker image to release artefacts

[cyphar]

At the moment we do not provide the source code for glibc, which is a bit of a problem. It wouldn't make any sense to compile glibc ourselves, so the simplest way to fulfill the LGPL requirements is to just provide an image that contains the debian metadata for how to get the source packages for the very specific glibc version we were built against.

This would also help somewhat with binary reproducibility.