serving: make the SPDK nvmeof target version-agnostic (stop recreating it on IMAGE_VERSION bump)

The nvmeof (spdk-target) service carried a versioned bootstrap mount
(type: image, source ${OCI_IMAGE}:${IMAGE_VERSION} -> /artifacts) plus an
IMAGE_VERSION env. Bumping IMAGE_VERSION therefore changed the service config,
so `docker compose up -d` recreated the container -> nvmf_tgt restarted ->
every connected host's NVMe/TCP root dropped at once. That is precisely the
fleet-wide outage the long-lived-target design exists to prevent.

Remove the /artifacts image-volume and IMAGE_VERSION env from nvmeof so its
config no longer depends on the version — compose leaves it running across a
bump. Bootstrap and rolling updates are already fully covered by spdk-loader
(stages the current release into ./spdk/releases and adds it online via
JSON-RPC); spdk-target.sh's seed block is guarded, so it just no-ops when
/artifacts isn't mounted. Verified nvmeof no longer references IMAGE_VERSION
(only spdk-loader does); compose YAML validates.

BENCH-VERIFY: bump IMAGE_VERSION + up -d with a host connected -> nvmeof
container not recreated, host root survives, loader adds the new version.

serving/docker-compose.yml

@@ -306,23 +306,21 @@ services:
       # Multi-version release store: each ./spdk/releases/<ver>/ holds that
       # version's rootfs.erofs + rootfs.hash + verity.json. Staged by the
       # spdk-loader (or spdk-ctl.sh on the host); reconciled into the target at
-      # (re)start and added/removed live thereafter. RW so the bootstrap seed +
-      # loader can write here. Adding a version does NOT recreate this service.
+      # (re)start and added/removed live thereafter. RW so the spdk-loader can
+      # stage here. Adding a version does NOT recreate this service.
       - ./spdk/releases:/releases
-      # Bootstrap convenience (optional): a single OCI release mounted as an
-      # image volume. spdk-target seeds it into /releases as ${IMAGE_VERSION} if
-      # not already staged — preserves the simple one-version `compose up` flow.
-      # Remove this block for a pure multi-version (spdk-ctl-managed) deployment.
-      - type: image
-        source: ${OCI_IMAGE}:${IMAGE_VERSION}
-        target: /artifacts
-        image:
-          subpath: artifacts
-      # Fallback (Docker < 28 / offline): - ${ARTIFACT_SRC}:/artifacts:ro
+      # NOTE: the target is deliberately VERSION-AGNOSTIC. It mounts NO
+      # ${IMAGE_VERSION} artifact image and carries no IMAGE_VERSION env, so a
+      # version bump never changes THIS service's config — `docker compose up -d`
+      # after an IMAGE_VERSION bump recreates the loader/http/dnsmasq but leaves
+      # nvmf_tgt running (recreating it would tear down EVERY host's NVMe/TCP root
+      # at once). Bootstrap AND rolling updates are the spdk-loader's job: it
+      # stages the current release into ./spdk/releases and adds it online via
+      # JSON-RPC. (spdk-target still seeds from /artifacts IF one is manually
+      # mounted + IMAGE_VERSION set — a legacy/standalone convenience, off here.)
       # Legacy single-version entrypoint (configure-at-start, restart-to-change):
       # - ./spdk-setup.sh:/spdk-setup.sh:ro
     environment:
-      IMAGE_VERSION: ${IMAGE_VERSION}    # bootstrap-seed label for /artifacts
       RELEASES_DIR: /releases
       RPC_SOCK: /var/run/spdk/spdk.sock  # shared with spdk-loader (-r / -s)
       SUBNQN_BASE: ${SUBNQN_BASE}        # per-version NQN base (falls back per release to verity.json .subnqn)