Remove cached .deb files to reduce image size

scripts/install-essentials

@@ -19,6 +19,8 @@ apt-get install -yqq --force-yes git
 rm /etc/apt/sources.list.d/stretch.list
 apt-get update -yqqq
+apt-get clean -yqq
+
 # Set UTF-8
 # http://stackoverflow.com/a/3182519/2137281
 LOC=$'LC_ALL=en_US.UTF-8\nLANG=en_US.UTF-8'