tests/docker: Add python3-venv and netcat to the debian-amd64 container

tests/docker: Add python3-venv and netcat to the debian-amd64 container

Without python3-venv, I get the following message when trying to
run the acceptance tests within the debian container:

 The virtual environment was not created successfully because ensurepip is not
 available.  On Debian/Ubuntu systems, you need to install the python3-venv
 package using the following command.
    apt-get install python3-venv
 You may need to use sudo with that command.  After installing the python3-venv
 package, recreate your virtual environment.

Let's do it as the message suggests.

And while we're at it, also add netcat here since it is required for
some of the acceptance tests.

Message-Id: <20200730141326.8260-2-thuth@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>