Lint for singularity image parsing

[ewels]

When running nf-core download you can sometimes get the following error log message:

ERROR    Cannot parse container string, skipping: ${            download.py:485
         workflow.containerEngine ==   

On the plus side, it doesn't fail. But the error message isn't super useful (would be good to log which file it came from).

More importantly, we should add a lint test to check pipelines that all container strings can be parsed, to avoid this happening in the first place.

[ErikDanielsson]

Closing due to merge of #1683