use csst as entrypoint

docker/Dockerfile

@@ -27,8 +27,8 @@ RUN --mount=type=cache,id=apt,target=/var/cache/apt \
 USER csst
 # copy VCS roots into image
 COPY --chown=csst:csst . /pipeline
-RUN mv pkg/csst-dag/app /pipeline/app \
-    && chmod +x /pipeline/app/csst \
+# make csst executable
+RUN chmod +x /pipeline/app/csst
 # add csst to PATH
 ENV PATH="/pipeline/app:${PATH}"