feat: make amd64 and arm64 image build targets (E00-S02-T07)

apps/server/Dockerfile

@@ -8,12 +8,14 @@
 # server answering `GET /health` with `{"status":"ok"}` (HTTP 200) on port
 # 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) and read-only root filesystem (T06) are
-# Compose-level concerns (see compose.yaml — the `db-data` volume mount and the
-# app service's `read_only: true` + `/tmp` tmpfs; this image is unchanged),
-# while multi-arch build targets (T07) remains a later E00-S02 task — out of
-# scope here. Since T05 the runtime stage drops root privileges (runs as the
-# image's non-root `node` user).
+# DB volume persistence (T04), read-only root filesystem (T06) and multi-arch
+# build targets (T07) are Compose-level concerns (see compose.yaml — the
+# `db-data` volume mount, the app service's `read_only: true` + `/tmp` tmpfs,
+# and its `build.platforms` list; this image is unchanged: both stages use the
+# official multi-arch node:24.19.0-bookworm-slim base and the build has no
+# native dependencies, so the amd64/arm64 targets need no image change). 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