Docker best practices

- Install and remove OS and Go build deps in a single layer
- Use COPY instead of ADD
- WORKDIR now resolves ENV variables, use one
- Use VOLUMES for configuration and datastore
- Added launcher script start-inbucket.sh
- Made docker-run.sh more powerful

etc/docker/defaults/greeting.html

@@ -0,0 +1,15 @@
+<p>Inbucket is an email testing service; it will accept email for any email
+address and make it available to view without a password.</p>
+
+<p>To view email for a particular address, enter the username portion
+of the address into the box on the upper right and click <em>View</em>.</p>
+
+<p>This instance of Inbucket is running inside of a <a
+  href="https://www.docker.com/" target="_blank">Docker</a> container.  It is
+configured to retain messages for a maximum of 3 days, and will enforce a limit
+of 300 messages per mailbox - the oldest messages will be deleted to stay under
+that limit.</p>
+
+<p>Messages addressed to any recipient in the <code>@bitbucket.local</code>
+domain will be accepted but not written to disk.  Use this domain for load or
+soak testing your application.</p>