The issue is that $PWD in the mount destination is evaluated on the host, but the destination path must be valid inside the container.
In your case:
dst=$PWD/my_folder/ → Host expands $PWD to /home/user/jail/, so destination = /home/user/jail/my_folder/ inside the container/home/user/jail/ in its filesystem/ (or another default), so ls lists a directory that does not contain your mount pointUse a relative path for the destination so it lands in the container's current working directory:
bashapptainer run --mount type=bind,src=$HOME/my_folder/,dst=my_folder/ ~/my_debian.sif ls
Now my_folder/ will appear in the container's working directory, and ls will show it.
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