explain why the COPY step exists

nextcloud/Containerfile

@@ -4,7 +4,10 @@ FROM docker.io/library/nextcloud:${TAG}
 RUN apt-get update \
     && apt-get install -y supervisor \
     && rm -rf /var/lib/apt/lists/* \
-    && mkdir /var/log/supervisord /var/run/supervisord
+    && mkdir -p /var/log/supervisord /var/run/supervisord 
+
+# We need to copy a file to the supervisord folder in /var/run so it exists when the container starts.
+COPY .keep /var/run/supervisord/
 
 COPY supervisord.conf /