[LXC] [TOS 6.x] Provide unprivileged containers index at repo.turris.cz for use with TurrisOS

Hi,

Since the issue with:

https://forum.turris.cz/t/omnia-armhf-lxc-armv7-32bit-deprecation-started/

https://www.linuxserver.io/blog/a-farewell-to-arm-hf

We no longer have unprivileged LXC containers in TOS 6.x because linuxcontainers.org stopped providing 32-bit ARM images. The Turris Team should step in and fill in that niche.

All what is it about is to provide index-user and index-user.asc at repo.turris.cz so the lxc-create tool won't complain when creating a container from template. When I look at the content of index-system and index-user at https://images.linuxcontainers.org/meta/1.0/ they are exactly the same so there might be no need to change the root filesystem of an container image.

Thank you in advance