Minified Rails/Unicorn startup failure

[kcq]

config.ru:1:in <main>': cannot load such file -- rack/builder (LoadError) from /usr/local/bundle/gems/unicorn-4.9.0/lib/unicorn.rb:48:ineval'
from /usr/local/bundle/gems/unicorn-4.9.0/lib/unicorn.rb:48:in block in builder' from /usr/local/bundle/gems/unicorn-4.9.0/lib/unicorn/http_server.rb:768:incall'
from /usr/local/bundle/gems/unicorn-4.9.0/lib/unicorn/http_server.rb:768:in build_app!' from /usr/local/bundle/gems/unicorn-4.9.0/lib/unicorn/http_server.rb:137:instart'
from /usr/local/bundle/gems/unicorn-4.9.0/bin/unicorn:126:in <top (required)>' from /usr/local/bundle/bin/unicorn:16:inload'
from /usr/local/bundle/bin/unicorn:16:in `

'

Reported in this blog post:
http://chrisstump.online/2016/02/23/docker-image-reduction-techniques-and-tools/

Similar Rails app:
https://github.com/cstump/docker_example

[kcq]

An interesting way to setup containers. It makes it trickier to resolve the issue. Still working on it :)