Push slicer-notebook image to dockerhub

[lassoan]

in addition of slicer/slicer-notebook:latest, we should have a proper tag of the form slicer/slicer-notebook:YYYYMMDD-SHA{7} like what is done in https://github.com/dockcross/dockcross

[lassoan]

@jcfr We still have not pushed an image to slicer/slicer-notebook. Could you just pull lassoan/slicer-notebook:2020-05-15-89b6bb5 and push that or would you like to build it yourself?

[lassoan]

@jcfr This is the issue that we talked about today at the Slicer weekly meeting.