Ensure that package managers clean up

Every package manager should clean up after themselves in order to keep
docker layers neat and tiny:

apt (Debian/Ubuntu package manager):

 - unneeded dependencies are cleared (autoremove)
 - caches are cleaned (clean)
 - package lists are deleted

yum (CentOS package manager),
zypper (OpenSuse package manager)

 - should clear caches after installing dependencies

pip (Python package manager),
apk (Alpine package manager)

 - should use no cache for installing dependencies

scripts/install-postgresql

@@ -10,4 +10,7 @@ echo "deb http://apt.postgresql.org/pub/repos/apt/ xenial-pgdg ${POSTGRES_VERSIO
 
 apt-get update
 apt-get install -y postgresql-client
+
 apt-get autoremove -yq
+apt-get clean -yqq
+rm -rf /var/lib/apt/lists/*