Add setup documentation for distrobox

[flibitijibibo]

This is mostly motivated by SteamOS including distrobox rather than toolbx:

ValveSoftware/SteamOS#1787

If possible, it'd be great to have distrobox documentation in places like this, so developers using a Steam Deck to compile/test don't have to manually install anything to get a convenient dev environment:

https://gitlab.steamos.cloud/steamrt/sniper/sdk#oci-images-for-docker-and-podman-

[smcv]

I haven't used distrobox, but I expect it'll be very similar to Toolbx?

[matte-schwartz]

Yes, the commands to set it up seem to be 1:1 as well (tested from SteamOS 3.7):

(deck@rog-ally-x ~)$ distrobox create -i registry.gitlab.steamos.cloud/steamrt/sniper/sdk sniper
Image registry.gitlab.steamos.cloud/steamrt/sniper/sdk not found.
Do you want to pull the image now? [Y/n]: y
Trying to pull registry.gitlab.steamos.cloud/steamrt/sniper/sdk:latest...
Getting image source signatures
Copying blob 3e57f6777ddb done   | 
Copying blob 4de64f9fb1e5 done   | 
Copying config 7d204682de done   | 
Writing manifest to image destination
7d204682deb2aeca5670cb229fd580d58ee6fdf191221ff04ce687c3d482dd68
Creating 'sniper' using image registry.gitlab.steamos.cloud/steamrt/sniper/sdk   [ OK ]
Distrobox 'sniper' successfully created.
To enter, run:

distrobox enter sniper

(deck@rog-ally-x ~)$ distrobox enter sniper
Starting container...                            [ OK ]
Installing basic packages...                     [ OK ]
Setting up devpts mounts...                      [ OK ]
Setting up read-only mounts...                   [ OK ]
Setting up read-write mounts...                  [ OK ]
Setting up host's sockets integration...         [ OK ]
Integrating host's themes, icons, fonts...       [ OK ]
Setting up distrobox profile...                  [ OK ]
Setting up sudo...                               [ OK ]
Setting up user's group list...                  [ OK ]
Setting up existing user...                      [ OK ]
Ensuring user's access...                        [ OK ]

Container Setup Complete!

[deck@sniper ~]$ cat /etc/os-release
PRETTY_NAME="Steam Runtime 3 (sniper)"
NAME="Steam Runtime"
VERSION_ID="3"
VERSION="3 (sniper)"
VERSION_CODENAME=sniper
ID=steamrt
ID_LIKE=debian
HOME_URL="https://store.steampowered.com/"
SUPPORT_URL="https://help.steampowered.com/"
BUG_REPORT_URL="https://github.com/ValveSoftware/steam-runtime/issues"
BUILD_ID="3.0.20250108.112707"
VARIANT=SDK
VARIANT_ID="com.valvesoftware.steamruntime.sdk-amd64_i386-sniper"