Remove apt lists to reduce the mailman-web image size.

web/Dockerfile

@@ -5,9 +5,10 @@ MAINTAINER Abhilash Raj
 ENV DEBIAN_FRONTEND=noninteractive
 
 RUN apt-get update \
-	&& apt-get install -y ruby-sass \
+	&& apt-get install -y --no-install-recommends ruby-sass \
 					   wget \
-					   postgresql-client
+					   postgresql-client \
+	&& rm -rf /var/lib/apt/lists/*
 
 RUN wget -O mailmanclient.zip https://gitlab.com/mailman/mailmanclient/repository/archive.zip?ref=master \
 	&& wget -O postorius.zip https://gitlab.com/mailman/postorius/repository/archive.zip?ref=master \