Improve the user guide documentation about containers & CWL/cwltool

[kinow]

Part of #219

These are recurrent questions about cwltool & CWL (the user guide being focused on cwltool, I think we can answer that in the guide).

• Can I use singularity? https://matrix.to/#/!RQMxrGNGkeDmWHOaEs:gitter.im/$-4wJsRgdBV7zuTN0eEki_BG1HFDks52Ei4iziA1YsV0?via=gitter.im&via=matrix.org&via=gottliebtfreitag.de
• We need to clarify the commands used in the container in the current docs - Clarity in "Running Tools Inside Docker" #119
• This issue is similar to the one above, where we need to improve the current docs - Some suggestions #190
• Can I run cwltool inside a container? https://matrix.to/#/!RQMxrGNGkeDmWHOaEs:gitter.im/$IU9NyGj_zQpy8Fx1oFxQ6tInUeRe5Ry9U9KYT6LYT2o?via=gitter.im&via=matrix.org&via=gottliebtfreitag.de

[kinow]

Created this issue as I don't think these will be addressed immediately in #226 👍

[swzCuroverse]

@kinow are these now covered? It looks like the first one is at least. https://www.commonwl.org/user_guide/faq.html#use-cwl-and-cwltool-with-singularity

[kinow]

@kinow are these now covered? It looks like the first one is at least. https://www.commonwl.org/user_guide/faq.html#use-cwl-and-cwltool-with-singularity

Ah, good catch. Yes, that's covered. But that was the only task of this issue that I believe to have been addressed.