Signal Desktop only supports Ubuntu 20.04 and newer, so we no longer
need to build against Ubuntu 16.04 to ensure compatibility. And
bullseye-slim is a smaller base image than the Ubuntu images, so if we
don't specifically need an Ubuntu package this should be an easy
improvement.
The only supported way to target an older glibc is to build against
that glibc; consequently, we need to build on an Ubuntu 16 system (or
similar) to target Ubuntu 16. This requires downloading second-party
versions of Clang and CMake, which are too old in the default Ubuntu
repository, as well as building our own Python.
Do all this in a new Dockerfile based on Ubuntu 16.04. This isn't as
rigorous as the Java "reproducible build" Dockerfile, since we're not
pinning the base image or the repositories we're fetching from, but
it's still an image with the environment and tools we need.
