I am new to docker and using apptainer for that.
the def file is: firstApp.def:
`Bootstrap: docker
From: ubuntu:22.04
%environment
    export LC_ALL=C
`
then I built it as follows and I want it to be writable (I hope I am not so naive), so I can install some packages later:
`apptainer build --sandbox --fakeroot firstApp.sif firstApp.def
`
now I do not know how to install Python3 (preferably, 3.8 or later).
I tried to add the following command lines to the def file:
`%post
   apt-get -y install update
   apt-get -y install python3.8 `
it raises these errors as well even without "apt-get -y install python3.8":
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
E: Unable to locate package update
FATAL:   While performing build: while running engine: exit status 100