Generate en_US.UTF-8 locale in Ruby base image

[baelter]

Summary

• Installs locales package and generates en_US.UTF-8
• Heroku apps migrated from slug to container deploys carry a LANG=en_US.UTF-8 config var that points to a locale not present on Debian slim
• Ruby silently falls back to US-ASCII encoding, breaking any code that reads files with non-ASCII bytes (Sass, ERB, etc.)
• With the locale installed, both LANG=C.UTF-8 and LANG=en_US.UTF-8 work correctly

Test plan

• Built image locally, verified locale -a includes en_US.utf8
• Verified LANG=en_US.UTF-8 ruby -e "puts Encoding.default_external" prints UTF-8