Proxmox, 4.2 LXC, container password reset

https://pve.proxmox.com/wiki/Manual:_pct

 pct exec <vmid> [<extra-args>]

    Launch a command inside the specified container.

    <vmid>     integer (1 - N)

              The (unique) ID of the VM.

    <extra-args> array

              Extra arguments as array

 

pct exec <vmid> passwd <username>