Since Xenial has the oldest glibc (2.23) of all the active distributions, this
attempts to construct a build image based on that. The Dockerfile here was built
by recursively expanding the `FROM` lines until reaching
`buildpack-deps:bionic-curl` and then changing that to `xenial`. Everything else
is the same.
Part of https://github.com/vector-im/riot-web/issues/13553