[SOLVED] IPv6, Docker, & Proxmox

Try this for your compose.

services:
  # python-matter-server
  matter-server:
    image: ghcr.io/home-assistant-libs/python-matter-server:stable
    container_name: matter-server
    restart: unless-stopped
    # Required for mDNS to work correctly
    network_mode: host
    security_opt:
      # Needed for Bluetooth via dbus
      - apparmor:unconfined
    volumes:
      # Create an .env file that sets the USERDIR environment variable.
      - ${USERDIR:-$HOME}/docker/appdata/matter-server/data:/data/
      # Required for Bluetooth via D-Bus
      - /run/dbus:/run/dbus:ro
    # If you adjust command line, make sure to pass the default CMD arguments too:
    #command: --storage-path /data --paa-root-cert-dir /data/credentials --bluetooth-adapter 0

    sysctls:    # Necessary for the container to receive the routes from the remote Thread network managed by my Google Nest Hub.
      net.ipv6.conf.eth0.accept_ra: 2
      net.ipv6.conf.eth0.accept_ra_rt_info_max_plen: 64
      net.ipv6.conf.eth0.forwarding: 1