Cache playwright install step on code changes

[ttilberg]

The Playwright install requires loading a small part of our application to sync the version with the ruby driver.

Since this doesn't require the host app's rails code, and instead only this gem's code, we could theoretically just separate the two "copy from build" lines, reorganize the dockerfile to:

COPY --from=build "${BUNDLE_PATH}" "${BUNDLE_PATH}"
RUN PLAYWRIGHT # install
COPY --from=build /rails /rails

This prevents common code changes from requiring a new playwright install. However, we also need to copy the gemfile for bundle exec to work:

COPY --from=build "${BUNDLE_PATH}" "${BUNDLE_PATH}"
COPY --from=build /rails/Gemfile /rails/Gemfile.lock ./
RUN PLAYWRIGHT # install
COPY --from=build /rails /rails

However, there's still a situation where the Gemfile might have changes that invalidate the playwright build, and the playwright version is very rarely going to change.

Instead, this PR prints a simple version file to rely on during the build step. Docker's build cache uses the content of that file to fingerprint, so the cache now only gets busted if the Playwright version actually changes.

Closes #63

[claude]

Claude Code Review

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[Copilot]

Pull request overview

This PR improves Docker build caching by decoupling the Playwright install layer from application code (and most dependency) changes, so Playwright is only reinstalled when the pinned Playwright version changes.

Changes:

• Persist the pinned Upright::PLAYWRIGHT_VERSION into /tmp/playwright_version during the build stage.
• Install Playwright in the final stage based on the copied version file (instead of running Ruby/Bundler there).
• Reorder final-stage layers so Playwright install happens before copying gems/app artifacts.

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[lewispb]

Good idea! Thanks @ttilberg