`cotainr --system=lumi-g ` uses a ROCm version that might not be compatible with all PyTroch versions

[gregordecristoforo]

We should probably mention somewhere in the course that cotainr --system=lumi-g uses a certain ROCm version and if someone tries to install a PyTorch version on top with a different ROCm version problems might occur.

Explicitly choosing the right base container via --base-image= resolves this issue