Fix 2 docker build problems: volume name too short, dubious ownership

[cweiske]

When trying to build on a Debian 12 installation with docker 20.10.24 I get multiple errors:

$ make
docker run --rm -v .:/var/www/en -w /var/www -u 1000:1000 php/doc-en
docker: Error response from daemon: create .: volume name is too short, names should be at least two alphanumeric characters.
See 'docker run --help'.
make: *** [Makefile:22: xhtml] Fehler 125

and

$ make
docker run --rm -v :/var/www/en -w /var/www -u 1000:1000 php/doc-en
configure.php on PHP 8.2.29, libxml 2.9.14

fatal: detected dubious ownership in repository at '/var/www/doc-base'
To add an exception for this directory, call:

       git config --global --add safe.directory /var/www/doc-base
doc-base/temp clean up FAILED.
make: *** [Makefile:22: xhtml] Fehler 1

The two commits in this PR fix both errors.

Related: https://github.com/php/doc-en/pull/4645