Oracle JDK-1.8 installation issue on Ubuntu-20.04-LTS
It seems that you blindly adding some PPA to your Dockerfile. This Oracle Java (JDK) 8 Installer PPA (DISCONTINUED) does not provide packages for 20.04 LTS (focal) - see http://ppa.launchpad.net/webupd8team/java/ubuntu/dists/ , also it is now marked as discontinued by its owners.
You have to replace this PPA by some other, for example this “Hellenic Schools Technical Support Team” team PPA provides needed package for Ubuntu 20.04 LTS .
So you have to put the following line into Dockerfile:
add-apt-repository ppa:ts.sch.gr/ppa