[Docker] Allow forcing version of PyPI package to use

Dockerfile

@@ -40,9 +40,10 @@ RUN   case ",$EXTRAS," in \
       esac
 
 # Install core
+ARG APP_VERISON=""
 RUN set -e; \
     mkdir -p /var/lib/aleksis/media /usr/share/aleksis/static /var/lib/aleksis/backups; \
-    eatmydata pip install AlekSIS-Core\[$EXTRAS\]
+    eatmydata pip install AlekSIS-Core\[$EXTRAS\]$APP_VERSION
 
 # Declare a persistent volume for all data
 VOLUME /var/lib/aleksis