Use a SWH_VER argument to make it easier to rebuild the image from that point

idea is be able to easily rebuild the images including latest swh packages
using docker image cache (prevent from having to run "docker build --no-cache").

images/base/Dockerfile

@@ -21,6 +21,7 @@ RUN mkdir -p /srv/softwareheritage/objects
 ENV SWH_CONFIG_FILENAME=/etc/softwareheritage/config.yml
 ENV LC_ALL=C.UTF-8
 
+ARG SWH_VER=unknown
 RUN export DEBIAN_FRONTEND=noninteractive && \
   apt-get update && \
   apt-get install -y \

images/web/Dockerfile

@@ -22,6 +22,7 @@ ENV SWH_CONFIG_FILENAME=/etc/softwareheritage/config.yml
 ENV LC_ALL=C.UTF-8
 ENV DJANGO_SETTINGS_MODULE=swh.web.settings.production
 
+ARG SWH_VER=unknown
 RUN export DEBIAN_FRONTEND=noninteractive && \
   apt-get update && \
   apt-get install -y --no-install-recommends \