The best option is to search for a Snap, Flatpak or Appimage version of the software and install it that way. For example:
sudo apt update
sudo apt install -y snapd
snap find krita
sudo snap install krita
snap run krita
Then add a menu item or launcher to a panel or key combination to run the command: snap run krita
Another option, which is less desirable, is to use Debian’s bullseye-backports repo (if you have Byzantium installed) to install recent software, and in my experience that is generally pretty safe and usually won’t screw up your system, because backports compiles newer versions of software to use the libraries in an older repo.
See: https://backports.debian.org/
The downside is that only a limited number of packages are included in the Debian backports repos, so you can’t upgrade the krita and podman packages that way.
The third option which is the least recommended is to use the Debian testingrepo. If you don’t mind taking a chance on ending up with a Frankenstein system that mixes Debian with PureOS which may be unstable and break stuff, then you can install the software from Debian’s testing repo.
Edit your sources list and add the Debian testing repo:
sudo nano /etc/apt/sources.list
At the bottom of the file add the following line:
deb http://ftp.us.debian.org/debian testing main
If you don’t mind installing proprietary stuff, use this line instead:
deb http://ftp.us.debian.org/debian testing main contrib non-free
Then, add a pinning preferences file to make sure that by default you install from PureOS’s Byzantium repo rather than Debian testing:
sudo nano /etc/apt/preferences.d/preferences
Add these lines:
Package: *
Pin: release a=byzantium
Pin-Priority: 900
Package: *
Pin: release a=testing
Pin-Priority: 500
If your /etc/apt/sources.list file uses “stable” instead of “byzantium”, then use:
Package: *
Pin: release a=stable
Pin-Priority: 500
Package: *
Pin: release a=testing
Pin-Priority: 100
The higher pinning number for byzantium/stable means that packages from that repo will be installed by default and you will only install packages from testing if you specify that repo. For more info, see: https://wiki.debian.org/AptConfiguration
Then, check your pinning policy to verify that everything is correct:
sudo apt-cache policy
Then update your package list:
sudo apt update
Now check what versions are available for krita and podman:
sudo apt-cache policy krita podman
Then, do a test run to see how many packages would be changed if you install from testing
sudo apt install -t testing --dry-run krita podman
If it looks like it will change a lot of packages in your system, then it probably isn’t a good idea to install from testing. If you don’t mind changing all those packages, then go for it:
sudo apt install -t testing krita podman