Singularity: linubrew folder disappers with --writable flag

[tmozgach]

Problem: usually a singularity container is read-only (brew install, apt-get don't work).
In order to edit a container, --writable flag is used. However, the brew command disappeared (see a sample script below) because linuxbrew folder doesn't exist in the folder /home!

singularity pull docker://tmozgacheva/orca:latest
singularity shell orca-latest.mg

Singularity orca-latest.img:~/sing> which brew
/home/linuxbrew/.linuxbrew/bin/brew

exit

singularity shell --writable orca-latest.mg
Singularity orca-latest.img:~/sing> which brew
Singularity orca-latest.img:~/sing> 
Singularity orca-latest.img:~/sing> brew --version
bash: brew: command not found

Question: could we adapt the singularity for our purpose because we need a function that edits a container !?

[tmozgach]

@sjackman if you are planing to play with Singularity, the above major issue that we faced.