Linux distribution and version

Gitlab Runner Running ubuntu:18.04 docker image

Flatpak version

1.4.2-flatpak1~bionic

Description of the problem

This is essentially issue #1326, however there are cases where it is impossible to run docker in privileged mode. Is it possible to use flatpak without a privileged docker?

Steps to reproduce

run the following in an unprivileged docker container:
flatpak-builder --disable-rofiles-fuse --repo="$FLATPAK_REPO" "$_FLATPAK_BUILD_FOLDER" "$FLATPAK_JSON_FILE".json;

The result will be:
bwrap: Creating new namespace failed: Operation not permitted