wrap-discord: Bind Discord RPC sockets
Discord provides a mechanism called "Rich Presence" that allows games to synchronize their state with Discord, e.g. showing status information in the user's profile.
To make this work from inside a Pressure vessel container, we need to
bind-mount the Discord IPC sockets.
They are expected to be called discord-ipc-, followed by a number that
ranges from zero up to nine.
They are usually located under XDG_RUNTIME_DIR, with TMPDIR, TMP,
TEMP and /tmp used as fallback alternatives.
Example games that are known to be using the Discord sockets are: VRChat, Among Us and osu!
Fixes: #102