xenial is no longer in LTS, so the docker image just doesn't build
anymore (lacks the letsencrypt ISRG root cert that classic.yarnpkg.com
uses).
Upgrade to the next LTS (bionic).
The libcurl change essentally reverts a change made when changing
back to xenial initially (xenial had curl3 and bionic has 4, so
installing a different libcurl version caused curl itself to be
removed).
Headlines: The Linux build image has been updated to bionic. This version of element will use a newer glibc which will drop support for some older systems.
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
