fix(docker): let non-root backend create its uploads dir (multer 2 eager mkdir)

[pmigueld]

Problem

Staging sync-server crash-looped (16× restarts) after the volume/init-script fix (#113) cleared the postgres issue. Real cause, from the container log:

EACCES: permission denied, mkdir '/app/uploads'
  at new DiskStorage (multer@2.2.0/storage/disk.js:20)
  at createOpenSppFieldRoutes (dist/routes/opensppFieldRoutes.js:260)

Two recent changes collide:

• multer 2.x eagerly mkdirSyncs a diskStorage destination at route-construction (startup), where multer 1.x did not.
• the backend container runs as the non-root node user.

WORKDIR /app is created root-owned, and COPY --chown=node:node only chowns the copied contents — not the /app directory itself. So node can't create /app/uploads (the OpenSPP-field route's upload dir, path.resolve(process.cwd(), "uploads")) → uncaught exception → crash loop. DB init succeeds first, which is why it looked DB-related.

Fix

In the backend stage of docker/Dockerfile, before USER node:

RUN mkdir -p /app/uploads && chown node:node /app /app/uploads

Gives node ownership of the workdir and pre-creates the uploads dir (multer's recursive mkdir then no-ops).

Scope / impact

Affects any fresh deploy of the backend image — including production / v2.1.0 (same Dockerfile + multer 2 + non-root). Recommend cherry-picking to main (or a 2.1.1) after this merges. Verified by redeploying staging.

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