Add possibility to skip css:build during assets:precompile

[ylecuyer]

Hello,

I was playing with a dockerfile like this:

FROM node:18-alpine as builder

WORKDIR /src
COPY package.json /src
COPY yarn.lock /src
RUN yarn install

COPY . /src

RUN yarn build:css

FROM ruby:3.0.4

WORKDIR /src

COPY Gemfile .
COPY Gemfile.lock .

RUN bundle install

COPY . .

COPY --from=builder /src/app/assets/builds/application.css /src/app/assets/builds/
RUN bundle exec rails assets:precompile

EXPOSE 3000

CMD ["bundle", "exec", "rails", "server", "-b", "0.0.0.0"]

And I was getting this error:

Step 16/18 : RUN bundle exec rails assets:precompile
 ---> Running in b268b243b355
sh: 1: yarn: not found
rails aborted!
cssbundling-rails: Command css:build failed, ensure yarn is installed and `yarn build:css` runs without errors or add SKIP_YARN

Tasks: TOP => assets:precompile => css:build
(See full trace by running task with --trace)

In this case it makes no sense as the css:build has already be done and the final docker image is free of node/yarn, that's why I'm proposing this CSS_BUILD=false similar to WEBPACKER_PRECOMPILE=false in https://github.com/rails/webpacker/blob/f1b06c7fd5bc4b7fc7128742d1078466b94af71f/lib/tasks/webpacker/compile.rake#L37 to skip the css:build.