By trial and error I worked this out while building a container running Weblogic Portal 10.3.6 (see my Dockerfiles in the 10.3.6updates branch of my fork of oracle/docker: https://github.com/kevinhooke/weblogic-docker/tree/10.3.6updates).
Even if you do a ‘USER oracle’ followed by a COPY or ADD, the files transferred into the image are still owned by root. This was causing me issues as the WLST script could not read the files owned by root (as you’d expect).
This issue and the justification is described in issue 6119. It doesn’t seem that this will be fixed, it seems like it’s ‘working as designed’.