Add io.github.vaulttec_dev.CosmicContainers

COSMIC panel applet to manage Docker containers, images, and volumes from
the panel. Requires host Docker (flatpak-spawn --host docker and
/var/run/docker.sock).

Upstream: https://github.com/vaulttec-dev/linux-pop-extension
Release commit: c61f1400902de089f6cd9b9cfc01a0cc4505bf35 (tag v0.1.0)

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: vaulttec-dev