Motivation

Build and runtime containers lacked a generated UTF-8 locale, which breaks or warns for apps that depend on locale-aware behavior (for example Python or Ruby). This aligns our default build containers with Heroku, which provides LANG=en_US.UTF-8 in their images by default.

Description

Generate en_US.UTF-8 in the Debian builder and runtime images at image build time so the locale is available without changing default process environment. Railpack still does not set LANG or LC_ALL; users opt in via deploy.variables or their platform's env config, as documented in the architecture overview and config reference.

Test

• mise run check
• Snapshot tests (updated pnpm-corepack-runtime-usage plan for current mise image tags)
• Rebuild builder/runtime images and run integration tests that exercise locale-sensitive builds (for example Ruby or Elixir examples) if validating the image change end-to-end