Still try..

Dockerfile_app

@@ -7,12 +7,15 @@ RUN pip install --upgrade pip -qq
 USER mesh
 
 VOLUME /home/mesh
-
 COPY ./meshtastic-matrix-relay /home/mesh/app
-COPY ./config.yaml /home/mesh/app/config.yaml
 
 RUN pip install -qq -r /home/mesh/app/requirements.txt
 
-RUN cp -r /home/mesh /mnt
+USER root
+RUN chown -R mesh:mesh /mnt
+#RUN bash -c "for f in /home/mesh/{*,.*}; do cp -r "$f" /mnt/; done"
+COPY ./config.yaml /mnt/app/config.yaml
+RUN chown -R mesh:mesh /mnt
+USER mesh
 
 WORKDIR /home/mesh