In the scenario:
./configure --prefix=<something_which_needs_su/sudo>
make domserver <as unpriviliged user which downloaded the tarball>
sudo make-installdomserver <as root>

We would copy the Nelmiodoc files as root, so the user can't run a make
{dist}clean without errors as we can't remove the folder as its owned by
root.

We now also install nelmiodoc together with composer in the base image
for CI:
- together with the needed dependencies
- make sure we get an autoload.php

See
DOMjudge#2303 (comment)
for the reasoning behind this.

This makes that we don't rerun the rule during
`domserver-install` if it already ran during `domserver`, so
that if you install as root, the local files are still owned
by the user who first ran `make domserver`, not root.

This does assume that if the directory `public/bundles/nelmioapidoc`
exists, then it contains the assets, and these won't automatically
get updated, for example, in the setting of a maintainer-install.