feat: run the app image as a non-root user (E00-S02-T05)

The runtime stage of apps/server/Dockerfile now drops root privileges with
'USER node' — the non-root user (uid/gid 1000) the official Node image ships
with — so the app container does not run with root privileges. The server
binds port 3000 (>= 1024) and only reads the root-owned files copied above,
so no extra user creation or ownership changes are required. compose.yaml
header updated: T05 is in scope; T06 (read-only rootfs) and T07 (multi-arch)
remain out of scope.

apps/server/Dockerfile

@@ -9,8 +9,10 @@
 # 3000, so the app container stays up and the health endpoint succeeds. The
 # Fastify 5 application shell (and the real HTTP API) lands in a later story;
 # DB volume persistence (T04) is a Compose-level concern (see compose.yaml —
-# this image is unchanged), while non-root/read-only hardening (T05/T06) and
+# this image is unchanged), while read-only root filesystem (T06) and
-# multi-arch targets remain later E00-S02 tasks — all out of scope here.
+# multi-arch build targets (T07) remain later E00-S02 tasks — all out of scope
+# here. Since T05 the runtime stage drops root privileges (runs as the
+# image's non-root `node` user).
 #
 # Image base: node:24.19.0-bookworm-slim (glibc Debian) per Technology-Stack
 # §5.4 — argon2 is a native dependency and musl/Alpine causes native-module
@@ -50,5 +52,13 @@ COPY --from=build /app/node_modules ./node_modules
 COPY --from=build /app/apps/server/dist ./apps/server/dist
 COPY --from=build /app/apps/server/package.json ./apps/server/package.json
 
+# T05: run as the image's non-root `node` user (uid/gid 1000, shipped with the
+# official Node image) so the app container does not run with root privileges.
+# The server binds port 3000 (>= 1024, no privileged port needed) and only
+# reads the root-owned application files copied above, so no extra user
+# creation or ownership changes are required. USER is the last instruction
+# before EXPOSE so every COPY above lands before the privilege drop.
+USER node
+
 EXPOSE 3000
 CMD ["node", "apps/server/dist/index.js"]

compose.yaml

@@ -1,4 +1,4 @@
-# EPPP Docker Compose baseline — [E00-S02-T01/T02/T03/T04]
+# EPPP Docker Compose baseline — [E00-S02-T01..T05]
 #
 # `docker compose up -d` starts both the database (PostgreSQL) and the
 # application (@personal-blog/server). Rollback: `docker compose down`.
@@ -18,8 +18,13 @@
 # `docker compose down` + `docker compose up -d` (recreate, which discards the
 # container filesystem). Reset the data with `docker compose down -v`.
 #
-# Explicitly out of scope for T01..T04 (land in later E00-S02 tasks):
+# Non-root execution (T05): the app image's runtime stage runs as the official
-#   - non-root execution (T05), read-only root filesystem (T06)
+# Node image's non-root `node` user (see apps/server/Dockerfile — `USER node`),
+# so the app container does not run with root privileges. No Compose-level
+# `user:` override is needed: the image's USER is inherited by the container.
+#
+# Explicitly out of scope for T01..T05 (land in later E00-S02 tasks):
+#   - read-only root filesystem (T06), multi-arch build targets (T07)
 #
 # All values have defaults so `docker compose up -d` works from a clean clone
 # without a .env file (a committed .env.example template lands in E00-S04).