Bump puma from 3.12.4 to 3.12.6 in /builder

Bumps [puma](https://github.com/puma/puma) from 3.12.4 to 3.12.6.
- [Release notes](https://github.com/puma/puma/releases)
- [Changelog](https://github.com/puma/puma/blob/master/History.md)
- [Commits](puma/puma@v3.12.4...v3.12.6)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

builder/Gemfile.lock

@@ -86,7 +86,7 @@ GEM
     nio4r (2.5.2)
     nokogiri (1.10.8)
       mini_portile2 (~> 2.4.0)
-    puma (3.12.4)
+    puma (3.12.6)
     rack (2.2.3)
     rack-proxy (0.6.5)
       rack