split Docker container content out from Setup

The setup page has periodically listed one or more of the unofficial
containers of IkiWiki. Now there are at least three, move their
description out to a separate page.

doc/containers.mdwn

@@ -0,0 +1,18 @@
+There are at least three unofficial container distributions of IkiWiki:
+
+ * [dgsb/ikiwiki](https://hub.docker.com/r/dgsb/ikiwiki/)
+   ([source on GitHub](https://github.com/dgsb/docker-ikiwiki)),
+   based on Debian Stretch, lighttpd and an internal ssh server
+
+ * [ankitrgadiya/ikiwiki](https://hub.docker.com/r/ankitrgadiya/ikiwiki)
+   ([source on GitHub](https://github.com/ankitrgadiya/docker-ikiwiki/)),
+   based on Debian stable and nginx,
+   by [[users/ankit]],
+   last updated 2018
+
+ * [opinionated-ikiwiki](https://quay.io/repository/jdowland/opinionated-ikiwiki)
+   ([source on GitHub](https://github.com/jmtd/opinionated-ikiwiki/)),
+   based on Debian 10, and lighttpd,
+   by [[users/Jon]]
+
+These can be used with a container runtime such as [Docker](https://www.docker.com/).

doc/setup.mdwn

@@ -7,9 +7,7 @@ This tutorial will walk you through setting up a wiki with ikiwiki.
 If you're using Debian or Ubuntu, ikiwiki is an <code><a href="http://www.debian.org/doc/manuals/debian-reference/ch02.en.html#_basic_package_management_operations">apt-get</a> install ikiwiki</code> away.
 If you're not, see the [[download]] and [[install]] pages.
 
-You may also want to try out a sandboxed [[Docker image|https://github.com/dgsb/docker-ikiwiki]]
+You may also want to try out IkiWiki in a [[container|containers]].
-with ikiwiki pre-installed which will use a volume to access a local git repository
-for the wiki content.
 
 ## Create your wiki