podman run with --userns=keep-id to mount volumes

Moving to /dangerzone was failing with insuficient permissions:

    Invalid JSON returned from container: PermissionError: [Errno
    13] Permission denied: '/dangerzone/page-3.rgb'

A previous approach was removed in commit 805222. It started with
root at first in a wrapper script and then dropped these
priviledges which running the script.

`--userns=keep-id` solves the mountpoint issues as it maps the user
starting the container is mapped in the container [1].

[1]: https://www.redhat.com/sysadmin/user-flag-rootless-containers

dangerzone/container.py

@@ -81,6 +81,7 @@ def convert(input_filename, output_filename, ocr_lang, stdout_callback):
     else:
         platform_args = []
         security_args = ["--security-opt", "no-new-privileges"]
+        security_args += ["--userns", "keep-id"]
 
     # drop all linux kernel capabilities
     security_args += ["--cap-drop", "all"]