A prebuilt image for amd64 and arm64, and the container no longer runs as root

Until now `docker compose up` built the image on the user's machine:
a compiler toolchain, every dev dependency, a few minutes of CPU — on a
Raspberry Pi considerably more. The compose files now pull
ghcr.io/tillheidrich/klarbild:1, built by .github/workflows/image.yml
for linux/amd64 and linux/arm64. `:1` follows every 1.x release, so an
update is `docker compose pull && docker compose up -d`. `build: .` is
one commented line away for anyone who prefers to build.

The Dockerfile has two stages now. The runtime stage carries the built
server, production dependencies and the migrations — no compiler, no
dev dependencies, no source tree. argon2 and sharp ship prebuilt
binaries for both architectures, so python/make/g++ are gone entirely.
It runs as the `node` user (uid 1000); a fresh named volume at /data
inherits that ownership.

Verified locally: image builds, boots against Postgres, applies all 17
migrations, prints the first-run password, passes its healthcheck,
accepts an upload as the unprivileged user and returns an A4 sheet that
measures 210.00 x 297.00 mm.

Dockerfile

@@ -1,22 +1,34 @@
-FROM node:22-bookworm-slim
+# Klarbild — two stages. The first builds, the second only runs: no compiler,
+# no dev dependencies, no source tree in the image that ends up on your server.
 
+FROM node:22-bookworm-slim AS build
 WORKDIR /app
-ENV NODE_ENV=production PORT=4321 HOST=0.0.0.0
-
-# Build tools for the native modules (argon2). sharp ships a prebuilt libvips.
-RUN apt-get update && apt-get install -y --no-install-recommends \
-      python3 make g++ ca-certificates \
-    && rm -rf /var/lib/apt/lists/*
-
 # The lockfile comes first and `npm ci` honours it, so the image is built from
 # the exact versions CI tested — not from whatever `^` resolved to today.
+# Native modules (argon2, sharp) ship prebuilt binaries for amd64 and arm64,
+# so no compiler is needed in either stage.
 COPY package.json package-lock.json ./
 RUN npm ci --no-audit --no-fund
-
 COPY . .
-RUN npm run build
+RUN npm run build && npm prune --omit=dev --no-audit --no-fund
+
+FROM node:22-bookworm-slim
+WORKDIR /app
+ENV NODE_ENV=production PORT=4321 HOST=0.0.0.0 STORAGE_DIR=/data
+
+COPY --from=build --chown=node:node /app/package.json ./
+COPY --from=build --chown=node:node /app/node_modules ./node_modules
+COPY --from=build --chown=node:node /app/dist ./dist
+# Read at runtime, on every boot.
+COPY --from=build --chown=node:node /app/migrations ./migrations
+
+# Not root. A fresh named volume mounted at /data takes over this ownership;
+# for a bind mount, give the directory to uid 1000 (`chown 1000:1000 ./data`).
+RUN mkdir -p /data && chown node:node /data
+USER node
 
 EXPOSE 4321
+VOLUME ["/data"]
 
 HEALTHCHECK --interval=30s --timeout=5s --start-period=40s --retries=3 \
   CMD node -e "fetch('http://127.0.0.1:'+(process.env.PORT||4321)+'/api/health').then(r=>process.exit(r.ok?0:1)).catch(()=>process.exit(1))"