[TMP] Add test to check if image is freshly built

base-notebook/Dockerfile

@@ -103,6 +103,7 @@ RUN mkdir "/home/${NB_USER}/work" && \
 # Do all this in a single RUN command to avoid duplicating all of the
 # files across image layers when the permissions change
 COPY --chown="${NB_UID}:${NB_GID}" initial-condarc "${CONDA_DIR}/.condarc"
+COPY --chown="${NB_UID}:${NB_GID}" test-file.txt /tmp/test-file.txt
 WORKDIR /tmp
 RUN set -x && \
     arch=$(uname -m) && \

base-notebook/test-file.txt

@@ -0,0 +1 @@
+test-content

tests/base-notebook/units/unit_check_file.py

@@ -0,0 +1,8 @@
+# Copyright (c) Jupyter Development Team.
+# Distributed under the terms of the Modified BSD License.
+
+from pathlib import Path
+
+file = Path("/tmp/test-file.txt")
+assert file.exists()
+assert file.read_text() == "test-content\n"