Since 5952928b, the python3-mock debian package is removed from the
Dockerfile used to build the Debian package. This prevent the Odoo
Debian package to be built.

The python3-mock debian package has an inderect dependency on
python3-setuptools which is necessary to build the package.

With this commit, the dependency is explicitely set in the Dockerfile.

closes odoo/odoo#49278

Signed-off-by: Christophe Monniez (moc) <moc@odoo.com>