build(web): standalone Docker image + drop Turbopack build cache (#4619)

The Docker production-stage opts into Next.js `output: 'standalone'` via a
BUILD_STANDALONE env flag, so it ships only the traced runtime (server.js +
hoisted node_modules + static/public) and runs `node server.js` instead of
pnpm over the full source tree. The flag — and `outputFileTracingRoot`,
which traces from the monorepo root so workspace packages are included — is
set only in the Dockerfile build stage. Every other path keeps its original
output: Tauri `export`, local `build-web`, dev, and the Cloudflare/OpenNext
deploy (which forces standalone itself via NEXT_PRIVATE_STANDALONE).

Disable the experimental `turbopackFileSystemCacheForBuild`: a build
interrupted mid-compile leaves a partial cache that the next build
mishandles, fanning out workers until it exhausts host RAM. Remove the
pull-request CI step that cached `.next/cache` for it, now unused.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/workflows/pull-request.yml

@@ -60,18 +60,6 @@ jobs:
           node-version: 24
           cache: pnpm
 
-      # Turbopack persists its build cache here (turbopackFileSystemCacheForBuild).
-      # The key is deterministic (no commit SHA) so every PR restores the same
-      # entry — in practice the one `main` last saved, which is the only cache
-      # sibling PRs can all see.
-      - name: cache Turbopack build cache
-        uses: actions/cache@27d5ce7f107fe9357f9df03efb73ab90386fccae # v5
-        with:
-          path: apps/readest-app/.next/cache
-          key: turbo-build-web-${{ runner.os }}-${{ hashFiles('pnpm-lock.yaml') }}
-          restore-keys: |
-            turbo-build-web-${{ runner.os }}-
-
       - name: install Dependencies
         working-directory: apps/readest-app
         run: |