docs: singularity sometimes uses system /tmp explanation

docs/project_info/faq.rst

@@ -689,3 +689,11 @@ Say you have forgotten how to use the various options starting ``force``, just t
 To activate this autocompletion permanently, put this line in ``~/.zshrc``.
 
 `Here <https://github.com/zsh-users/zsh-completions/blob/master/zsh-completions-howto.org>`_ is some further reading.
+
+How can I avoid system /tmp to be used when combining singularity and conda?
+------------------------------------------------------
+
+When using both singularity and conda the idea is that inside the singularity container the conda environment is being installed.
+Some singularity instances are set to share the system /tmp with the containers.
+This can lead to unexpected behaviour where the system /tmp gets full.
+To stop this behaviour you'd have to run singularity with the ``--contain`` option.