How can I make jenkins run "pip install"?

The fact that you have to run use sudo to run pip is a big warning that your virtual environment isn't working. The build output shows that pip is installing the requirements in your system site-packages directory, which is not the way virtualenv works.

Your build script doesn't actually preserve the activated virtual environment. The environment variables set by the activate script are set in a child bash process and are not propagated up to the build script. You should source the activate script instead of running a separate shell:

virtualenv venv --distribute
. venv/bin/activate 
pip install -r requirements.txt
python tests.py

If you're running this as one build step, that should work (and install your packages in venv). If you want to add more steps, you'll need to set the PATH environment variable in your other steps. You're probably better off providing full paths to pip and python to ensure you're not dependent on system package installations.

Try using

stage('test') {
     agent {
          docker {
               image 'qnib/pytest'
          }
     }
     steps {
          sh 'virtualenv venv && . venv/bin/activate && pip install -r requirements.txt && python tests.py'
     }
}