fix(docker-dev): self-bootstrap schema via one-shot migrator (fixes fresh-volume quirks)

Adds a 'migrator' Dockerfile stage + Compose service that runs 'dotnet ef
database update' once on bring-up, so a fresh SQL volume gets the schema with no
operator step (quirk 1). cluster-seed + every host node depend on it via
service_completed_successfully, so the seed never races an in-progress migration
(quirk 2). Host build pinned to target: runtime (the migrator is now the last
stage). entrypoint + README updated; the manual 'dotnet ef' first-time step is
gone. Verified: down -v + up --build self-bootstraps (migrator+seed exit 0,
6 nodes up), deploy Sealed 6/6.

docker-dev/Dockerfile

@@ -1,6 +1,7 @@
 # Multi-stage build of OtOpcUa.Host targeting linux-x64. Used by docker-dev/docker-compose.yml
 # to spin six host containers (central-1, central-2, site-a-1, site-a-2, site-b-1, site-b-2) from a single image —
 # Compose drives OTOPCUA_ROLES + Cluster:* env per container to differentiate them.
+# A separate `migrator` stage (below) applies EF migrations once on bring-up.
 
 FROM mcr.microsoft.com/dotnet/sdk:10.0 AS build
 WORKDIR /src
@@ -18,3 +19,18 @@ EXPOSE 4053
 EXPOSE 4840
 
 ENTRYPOINT ["dotnet", "OtOpcUa.Host.dll"]
+
+# ── Migrator (one-shot) ──────────────────────────────────────────────────────
+# Applies EF Core migrations to the ConfigDb so a fresh SQL volume gets the schema
+# with no operator step. docker-dev compose runs this once, before cluster-seed +
+# the host nodes (they depend on it via service_completed_successfully). The host
+# nodes deliberately do NOT auto-migrate (production owns schema changes), so this
+# rig-only stage carries that responsibility. The connection comes from the
+# OTOPCUA_CONFIG_CONNECTION env var read by DesignTimeDbContextFactory.
+FROM build AS migrator
+RUN dotnet tool install --global dotnet-ef --version 10.0.7
+ENV PATH="${PATH}:/root/.dotnet/tools"
+WORKDIR /src
+ENTRYPOINT ["dotnet", "ef", "database", "update", \
+    "--project", "src/Core/ZB.MOM.WW.OtOpcUa.Configuration", \
+    "--startup-project", "src/Core/ZB.MOM.WW.OtOpcUa.Configuration"]