Add non-root variant to Dockerfile for dual image publishing

Dockerfile

@@ -7,11 +7,75 @@ WORKDIR /src
 COPY [".", "./"]
 RUN dotnet build "./src/Service/Azure.DataApiBuilder.Service.csproj" -c Docker -o /out -r linux-x64
 
-FROM mcr.microsoft.com/dotnet/aspnet:8.0-cbl-mariner2.0 AS runtime
+# ---------------------------------------------------------------------------
+# Common runtime base.
+#
+# Not intended as a final build target. Both the `runtime` (root, default)
+# and `runtime-nonroot` (non-root, scanner-friendly) variants derive from
+# this stage so the shared setup stays in one place.
+# ---------------------------------------------------------------------------
+FROM mcr.microsoft.com/dotnet/aspnet:8.0-cbl-mariner2.0 AS runtime-base
 
 COPY --from=build /out /App
 # Add default dab-config.json to /App in the image
 COPY dab-config.json /App/dab-config.json
 WORKDIR /App
 ENV ASPNETCORE_URLS=http://+:5000
+EXPOSE 5000
 ENTRYPOINT ["dotnet", "Azure.DataApiBuilder.Service.dll"]
+
+# ---------------------------------------------------------------------------
+# Non-root variant. Build explicitly with:
+#   docker build --target runtime-nonroot -t <repo>:<version>-nonroot .
+#
+# Runs as the "app" user that ships with the mcr.microsoft.com/dotnet/aspnet
+# base image (UID/GID 1654 on the cbl-mariner variant; 64198 on the
+# Debian/chiseled variants). DAB does not require root, and declaring USER
+# explicitly sets the image's Config.User field so image scanners
+# (e.g. Checkmarx One) that require a non-root user in the final stage are
+# satisfied.
+#
+# Safeguards applied to minimize the chance of runtime breakage:
+#   * `chown -R app:app /App` so any feature that writes to a relative path
+#     under WORKDIR works (e.g. runtime.telemetry.file with the default path
+#     "logs/dab-log.txt", or any future relative-path artifact). File modes
+#     are unchanged - only ownership.
+#   * Pre-create /App/logs so the documented default file-sink path works
+#     with no additional volume/permission setup.
+#   * Default port stays at 5000, which is above 1024, so binding works
+#     without CAP_NET_BIND_SERVICE. Users overriding ASPNETCORE_URLS to a
+#     privileged port (<1024) must add `--cap-add=NET_BIND_SERVICE` to
+#     `docker run` or front DAB with a reverse proxy.
+#
+# Notes for consumers of this image:
+#   * Host bind-mounts (config, logs, certs, etc.) must be readable - and
+#     writable, if DAB needs to write them - by UID 1654 on the host.
+#     Either `chown -R 1654:1654 /host/path` or, in Kubernetes, set
+#     `securityContext.fsGroup: 1654`.
+#   * `docker exec` defaults to the `app` user. Use `docker exec --user 0`
+#     for administrative actions inside a running container.
+#   * Downstream Dockerfiles (FROM <this image>) that need to install
+#     packages or write outside /App should add `USER 0` before those
+#     instructions, then restore `USER app` at the end.
+# ---------------------------------------------------------------------------
+FROM runtime-base AS runtime-nonroot
+
+RUN mkdir -p /App/logs && chown -R app:app /App
+USER app
+
+LABEL org.opencontainers.image.title="Data API builder (non-root)" \
+      org.opencontainers.image.description="Data API builder running as the non-root 'app' user (UID 1654 on cbl-mariner)." \
+      org.opencontainers.image.source="https://github.com/Azure/data-api-builder"
+
+# ---------------------------------------------------------------------------
+# Default (root-running) variant. Build with either:
+#   docker build -t <repo>:<version> .                       # no --target needed
+#   docker build --target runtime -t <repo>:<version> .
+#
+# This is the LAST stage in the file, so a plain `docker build` with no
+# --target argument produces this image. Keeping the root-running variant
+# as the default preserves backwards compatibility with the previously
+# published image - existing users see no behavior change. The non-root
+# variant is opt-in via `--target runtime-nonroot`.
+# ---------------------------------------------------------------------------
+FROM runtime-base AS runtime