Document keyboard udev rule

[joonas-fi]

Why is it a hardlink: tl;dr: hotplug support.

If you don't want to run screen-server with --privileged (you shouldn't), and you don't want to bind the entire /dev/input (violates principle of least privilege), you're left with having to map a directory that contains the sole event device. It has to be directory, because device nodes come and go (hotplug). i.e. plug creates inode 1 -> disconnect -> plug creates inode 2. If you'd naively map --device /dev/input/logitech and start a container, during the whole lifecycle of the container it'd refer to inode 1.

Here's my /etc/udev/rules.d/80-screen-server.rules:

# Logitech should be visible for screen-server

ACTION=="add", KERNEL=="event[0-9]*", ATTRS{name}=="Logitech K400", RUN+="/bin/sh -c 'mkdir -p /dev/input/screen-server && ln $devnode /dev/input/screen-server/logitech'"
ACTION=="remove", KERNEL=="event[0-9]*", ATTRS{name}=="Logitech K400", RUN+="/bin/sh -c 'rm /dev/input/screen-server/logitech'"

And docker-compose file:

version: "3.5"
services:
  screens:
    labels:
      traefik.frontend.rule: Host:screens.mydomain.com
    environment:
      SCREEN_1: "5900,800,1280,Galaxy Tab 2,/dev/input/screen-server/logitech"
    image: fn61/screen-server:20201218_1758_3beeef8d
    shm_size: 512M # Firefox and Chrome crash on default (64 M) if website is heavy in any way
    ports:
      - "5900:5900"
    devices:
    - "/dev/input/screen-server"
    networks:
      default: null
networks:
  default:
    external:
      name: mynetwork