addons: Remove docker

Used it only or twice in the past

Author: fgrehm

CHANGELOG.md

@@ -1,8 +1,10 @@
 ## [1.0.0](https://github.com/fgrehm/devstep/compare/v0.4.0...next) (unreleased)
 
 BREAKING CHANGES:
+
   - baseimage: Removed PostgreSQL and MySQL clients
   - baseimage: Removed tmux
+  - addons: Removed `docker`
   - buildpacks: Removed `bats` and `phantomjs`
 
 IMPROVEMENTS:

docs/addons/docker.md

@@ -128,7 +128,6 @@
               <li class="dropdown">
                 <a class="dropdown-toggle" data-toggle="dropdown" href="#">Addons<span class="caret"></span></a>
                 <ul class="dropdown-menu">
-                  <li><a tabindex="-1" href="addons/docker">Docker</a></li>
                   <li><a tabindex="-1" href="addons/memcached">Memcached</a></li>
                   <li><a tabindex="-1" href="addons/postgresql">PostgreSQL</a></li>
                   <li><a tabindex="-1" href="addons/redis">Redis</a></li>