Keep in mind that any output file (--log, --html, --json etc.) will be created in the container. If you wish to have this created in a local directory you can mount a volume into the container and change the output prefix where the container user has write access to, e.g.:
which writes the output to ``/tmp/example.com_p443-<date>-<time>.html.`` The uid/gid is the one from the docker user but normally the file is 644. testssl.sh's docker container uses a non-root user (usually with user/groupid 1000:1000).