Add TAILWINDCSS_SKIP_BUILD to skip the Tailwind build during asset compilation

[cgmoore120]

Summary

The gem unconditionally enhances assets:precompile (and the test prepare task) with tailwindcss:build, so the Tailwind build runs automatically before assets are digested or tests are prepared. There is currently no supported way to opt out of this.

This adds a TAILWINDCSS_SKIP_BUILD environment variable: when set to any non-blank value, the automatic enhancement is skipped. Default behavior is unchanged when the variable is unset or blank, so this is fully backward-compatible.

Motivation

A few situations where running the Tailwind build automatically is undesirable:

1. Pre-built / committed CSS — Some apps build and commit app/assets/builds/tailwind.css ahead of time and don't want assets:precompile to rebuild (and potentially overwrite) it.
2. CI / Docker layer caching — When the build output is produced in an earlier, cached step, re-running it during assets:precompile wastes time and defeats layer caching.
3. Cross-architecture Docker builds — When building an image for a different architecture (e.g. an arm64 host producing an amd64 image via emulation), the standalone CLI can fail to produce correct output. There is a known float-truncation issue in this area on Apple Silicon / Rosetta with Bun (see oven-sh/bun#19677 and tailwindlabs/tailwindcss#17728). This isn't a bug in this gem, but being able to skip the automatic build lets users build the CSS in a context where it works and skip it where it doesn't.

Precedent

The gem already reads an environment flag in the same idiom: ENV["TAILWINDCSS_DEBUG"].present? in lib/tailwindcss/commands.rb. This change uses the same .present? check for consistency.

Changes

• Wrap the trailing enhance(...) block in lib/tasks/build.rake in unless ENV["TAILWINDCSS_SKIP_BUILD"].present?.
• Document the variable in the README near the existing build/TAILWINDCSS_DEBUG docs.
• Add a CHANGELOG entry.
• Add a test verifying that tailwindcss:build is attached to assets:precompile and test:prepare by default, that it is not attached when TAILWINDCSS_SKIP_BUILD is set, and that a blank value does not skip the build.

Testing

bundle exec rake test passes (30 runs, 214 assertions, 0 failures, 0 errors, 0 skips).

[flavorjones]

@cgmoore120 Thanks for the pull request. I'm not opposed to doing something to omit tailwind compilation, but I do want to make sure we're fixing it in the right place. I am very curious why the tailwind assets are being treated differently from other assets with respect to points 1 and 2 above:

1. Why commit generated assets to source control?
2. What is the advantage to generating tailwind assets in an earlier image layer?

I don't think these are common idioms with Rails, but if I'm wrong then my instinct would be to start a discussion on whether the assets:precompile task itself should be skippable. Do these questions make sense given that context?