Remove building from docker-compose and move that to a seperate script.

I want to be able to distribute docker-compose.yaml without any dependencies. In
order to do that, I should make it pull stable images from hub.docker.com
instead of trying to build them locally. This will avoid users running untested
code from the github repo instead of using the tested version on docker hub.

build.sh

@@ -0,0 +1,11 @@
+#!/bin/bash
+
+set -e
+
+# Use this script to build docker images.
+
+DOCKER=docker
+
+
+$DOCKER build -t maxking/mailman-core core/
+$DOCKER build -t maxking/mailman-web web/

docker-compose.yaml

@@ -2,8 +2,6 @@ version: '2'
 
 services:
   mailman-core:
-    build:
-      context: ./core
     image: maxking/mailman-core:latest
     container_name: mailman-core
     hostname: mailman-core
@@ -23,8 +21,6 @@ services:
         ipv4_address: 172.19.199.2
 
   mailman-web:
-    build:
-      context: ./web
     image: maxking/mailman-web:latest
     container_name: mailman-web
     hostname: mailman-web