I had a user outside with a different name than the user inside. Both had the same UID and so I could write on "both sides of the wall" because file permissions had the same UID in & out of docker on a bind-mount volume.
It does not work anymore with the current versions of docker : outside the bind/mount directory belongs to myUser (UID=1000), and inside it belongs to root (UID=0).
I conducted a search to investigate recent changes in Docker's handling of file permissions and ownership for bind mounts but did not find specific information about any recent updates or changes made by Docker in this regard. To address this issue, I suggest try these several options:
RUN adduser --uid 1000 myUser and
USER myUser in your Dockerfile.
You can find useful information in this article: