feat: multi-stage Dockerfile to reduce addon image size

[Copilot]

The addon installs slowly because the final image was built directly FROM ghostfolio/ghostfolio, inheriting yarn, build tooling, and OS packages that aren't needed at runtime (~302 MB compressed before our layers).

Dockerfile — multi-stage build

• Stage 1 (ghostfolio-source): ghostfolio/ghostfolio used as source only — nothing lands in the final image except what's explicitly copied
• Stage 2 (final): node:22-bookworm-slim — the same clean base the upstream ghostfolio image itself uses; strips yarn, build tools, and all upstream cruft
• COPY --from=ghostfolio-source --chown=node:node /ghostfolio /ghostfolio — only app files cross the stage boundary
• --no-install-recommends on apt-get install; redis → redis-server (drops unused CLI); curl and openssl added explicitly (previously inherited from base)

Run script

• yarn start:production → exec /ghostfolio/entrypoint.sh
• The upstream entrypoint does the same work: prisma migrate + seed + node main, without yarn
• exec makes node a direct child of s6, improving signal handling and eliminating the yarn process overhead; yarn is no longer required in the image

BUILD_FROM build arg and build.json are unchanged — the ghostfolio source image is still pinned per-architecture as before.