improve reproducibility

to avoid interaction with ~/.local which is the python cache
for package i use PYTHONNOUSERSITE.
it could interact during the build phase but more important
at runtime (if we have a more recent lib in our cache than in image)

singularity/Singularity

@@ -22,6 +22,9 @@ from: ubuntu:bionic
     #################################
     #        installing craw        #
     #################################
+    export PYTHONNOUSERSITE=1
+    # don't add user site-packages directory to sys.path.
+    # this avoid to interact with ~/.local
     cd /usr/local/src
 	git clone https://gitlab.pasteur.fr/bneron/craw/
 	cd craw
@@ -37,6 +40,12 @@ from: ubuntu:bionic
 	apt-get autoremove -y
     apt-get clean -y
 
+%environment
+    # to be reproducible we should nevr search
+    # in python user cache which is the user home
+    # and is mounted automically
+    export PYTHONNOUSERSITE=1
+
 %test
     /usr/bin/python3 /craw/tests/run_tests.py -vv