From what I understand, bubblewrap is supposed be configured by passing flags from the command line. It seems that the way to “configure” bubblewrap is to create wrapper scripts. For example make /usr/local/bin with the following contents

#!/usr/bin/bash
bwrap --flags-and "arguments" steam

As it’s not very practical to figure out a good sandbox from scratch for each and every program you use, you probably want to find scripts from other users or tools that build on top of bubblewrap and are bundled with profiles. The wiki article has examples of both.

Here’s how I run Firefox, for instance:

#!/bin/zsh

function r { for p in $@; do [[ -e $p ]] && echo --ro-bind-try $p $p; done; }
function w { for p in $@; do [[ -e $p ]] && echo --bind-try $p $p; done; }
function ln { echo --symlink $1 $2; }
function wdev { for p in $@; do echo --dev-bind-try $p $p; done; }

bwopt=(
  --unshare-pid --unshare-uts --unshare-ipc --unshare-cgroup

  --proc /proc --dev /dev --tmpfs /dev/shm --mqueue /dev/mqueue

  $(wdev /dev/dri /dev/v4l /dev/video*)
  $(r /sys/{dev,devices,bus/pci})

  --dir /var/tmp --dir /run/lock
  $(ln ../run /var/run) $(ln ../run/lock /var/lock)
  $(w /tmp/.{X11-unix,ICE-unix})

  $(r /usr/lib) $(ln usr/lib /lib64) $(ln lib /usr/lib64)
  $(r /usr/share)
  $(r /var/{cache/fontconfig,lib/dbus/machine-id})

  $(r /etc/{passwd,group,nsswitch.conf,resolv.conf,hosts,gai.conf,ld.so*})
  $(r /etc/{localtime,lsb-release,machine-id})
  $(r /etc/{ca-certificates,ssl})
  $(r /etc/{dconf,fonts,gtk-*,host.conf,xdg,mime.types,pulse})
 
  $(r ${XAUTHORITY} ${DBUS_SESSION_BUS_ADDRESS/unix:path=})
  $(w ${XDG_RUNTIME_DIR}/{ICEauthority,dconf,pulse,gvfsd,wayland-*,p11-kit,flatpak-info})

  $(w ~/.{mozilla,cache/mozilla})
  $(r ~/.cache/{fontconfig,mesa_shader_cache})
  $(r ~/.config/{dconf,fontconfig,user-dirs.dirs,gtk-*,mimeapps.list,pulse})
  $(r ~/.{fonts,local/share/{themes,icons}})

  $(w ~/down /tmp/swap)
)

exec nice \
  systemd-run --quiet --user --scope --slice=firefox.slice \
  bwrap --args 9 9< <(printf $'%s\0' $bwopt) \
  -- /usr/lib/firefox/firefox $@

Using this for about 5 years. Ran strace on a session to see what to allow access to. It’s got full access to /lib and too much access to /sys b/c I’m lazy, but it can not see any executables or most of ~.

I’m using something similar whenever I want to precisely isolate a program.

Comment federation is glitched, yo.

This, that. But, but, but.

¯\(°_o)/¯

I don’t have any experience with Bubblewrap. Is it what people tend to use instead of its alternatives? Have you had a look at Firejail? I think it does what you are trying to achieve and has a lot of these preconfigured scripts for a variety of the applications you might use (they call them profiles). https://wiki.archlinux.org/title/Firejail From the archwiki:

Most users will not require any custom configuration and can proceed to #Usage.

Firejail uses profiles to set the security protections for each of the applications executed inside of it - you can find the default profiles in /etc/firejail/application.profile. Should you require custom profiles for applications not included, or wish to modify the defaults, you may place new rules or copies of the defaults in the ~/.config/firejail directory. You may have multiple custom profile files for a single application, and you may share the same profile file among several applications.

If firejail does not have a profile for a particular application, it uses its restrictive system-wide default profile. This can result in the application not functioning as desired, without first creating a custom and less restrictive profile.

It also has support for use in conjunction with Apparmor: https://wiki.archlinux.org/title/Firejail#Enable_AppArmor_support

Note: A lot of applications won’t have any read or write access anywhere but /home/$USER/Downloads. So one example from me would be that I copied the Firefox profile from /etc/firejail/firefox.local to /home/$USER/firejail/firefox.local and edited the latter to allow Firefox access to /home/$USER/Pictures for the sake of convenience when saving a picture.

Just my two cents in case you are not dead set on Bubblewrap.