Detect when Singularity isn't properly configured to mount local folders

I am trying to define a `CommandLineTool` which is either available via a docker container (pulled from our private repository) or directly from a Singularity image file on the local machine or from an SFTP server but could not find out how to do it.
In the docs I found a hint that Singularity is supported but could not find a field to describe it.

What I want to do is running the tool inside a Docker container OR using a Singularity image with a given path.

Can anyone point me into the right direction?