Remove apt cached data (reduce image size)

dockerfile

@@ -12,6 +12,5 @@ RUN set -ex; \
     --update-no-dev \
     --no-interaction \
     --optimize-autoloader \
-    require roundcube/carddav ;
-
-
+    require roundcube/carddav ; \
+    rm -rf /var/lib/apt/lists/*