Shift creation and activation of venv back to Dockerfile

Making a venv at /venv requires root permissions as well, as it turns
out. Using it will be as well so we chown it.

.ci/Dockerfile

@@ -27,3 +27,7 @@ COPY install-sys-pkgs.sh .
 # jq and iproute2 are used to find the interface IPv4 address
 RUN apt-get update && apt-get -y --no-install-recommends install libpython3-dev python3-venv jq iproute2
 RUN ./install-sys-pkgs.sh
+
+RUN python3 -m venv /venv
+RUN chown -R +1000:+1000 /venv
+ENV PATH=/venv/bin:$PATH

Jenkinsfile

@@ -44,8 +44,7 @@ pipeline {
   stages {
     stage('Install dependencies') {
       steps {
-        sh 'python3 -m venv /venv'
-        sh 'PATH="/venv/bin:$PATH" py-requirements.sh'
+        sh '.ci/py-requirements.sh'
 
       }
     }