chore(linea-ens-app): refactor Dockerfile to reduce image size

[alainncls]

Fixes #361

From 8.18GB to 294MB (28 times smaller!)

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Note

Refactors packages/linea-ens-app Docker build to a slim multi-stage Node 22 Alpine image using pnpm and Next.js standalone output, adds a repo-level .dockerignore, updates Next config for standalone/tracing, and tweaks stylelint config.

• Build/Docker:
  • Replace legacy Dockerfile with multi-stage node:22-alpine build using Corepack/pnpm, cached installs, and non-root runtime user.
  • Build Next.js app as output: 'standalone'; copy standalone server, static assets, and public/ into runner; start via node packages/linea-ens-app/server.js.
  • Limit workspace install by copying only required package.jsons and lockfile; add system deps via apk.
  • Add top-level .dockerignore to exclude dev/test artifacts and unrelated packages from the build context.
• Next.js config (packages/linea-ens-app/next.config.mjs):
  • Enable output: 'standalone' and set outputFileTracingRoot to monorepo root for pnpm; minor cleanup of generateBuildId.
• Lint (.stylelintrc.json):
  • Minor rule/format updates; set no-descending-specificity to null.

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