Bump rack from 2.0.8 to 2.2.3 in /docker (#78)

Bumps [rack](https://github.com/rack/rack) from 2.0.8 to 2.2.3.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/master/CHANGELOG.md)
- [Commits](rack/rack@2.0.8...2.2.3)

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docker/Gemfile.lock

@@ -10,7 +10,7 @@ GEM
       erubis
       que (~> 0.8)
       sinatra
-    rack (2.0.8)
+    rack (2.2.3)
     rack-protection (2.0.7)
       rack
     sequel (5.5.0)