I have Ubuntu 18:04 Bionic installed in a Singularity container.(*)
Via the Singularity definition file I try to add the Okular PDF viewer with the command line
apt-get install -y okular
However I get the message "E: Unable to locate package okular" (no other message: No 404 for URL or similar).
From answers to Installing okular via apt-get on bionic gives me - Ask Ubuntu I took the advice to add the universe
repo, but apt-add-repository
does not seem to be part of the minimal bionic image (I get the message "add-apt-repository: not found").
How can I install okular on this ubuntu image?