Run the sync server image as a numeric UID/GID; explain unwritable /data

The image now runs as USER 10001:10001 with a matching group, so
Kubernetes can verify runAsNonRoot and fsGroup: 10001 grants it a mounted
volume. When the database can't be opened because its directory isn't
writable, the server says so and how to fix it, instead of SQLite's bare
"unable to open database file". The README covers the pod securityContext.

crates/mtgc-server/Dockerfile

@@ -13,10 +13,13 @@ FROM debian:bookworm-slim
 RUN apt-get update \
  && apt-get install -y --no-install-recommends ca-certificates \
  && rm -rf /var/lib/apt/lists/* \
- && useradd --system --uid 10001 --home /data mtgc \
- && mkdir /data && chown mtgc /data
+ && groupadd --system --gid 10001 mtgc \
+ && useradd --system --uid 10001 --gid 10001 --home /data mtgc \
+ && mkdir /data && chown 10001:10001 /data
 COPY --from=build /mtgc-server /usr/local/bin/mtgc-server
-USER mtgc
+# Numeric, so Kubernetes can verify `runAsNonRoot` (it can't resolve names),
+# and with a fixed group so `fsGroup: 10001` grants a mounted volume to it.
+USER 10001:10001
 VOLUME /data
 ENV MTGC_DB=/data/mtgc-server.db \
     MTGC_BIND=0.0.0.0:8080