Singularity installation instructions

[susheelbhanu]

Hi,

I'm trying to install nf-core/scflow via singularity and am running into issues as follows:

>singularity pull scflow.sif docker://nfcore/scflow
WARN[0000] "/run/user/6154" directory set by $XDG_RUNTIME_DIR does not exist. Either create the directory or unset $XDG_RUNTIME_DIR.: stat /run/user/6154: no such file or directory: Trying to pull image in the event that it is a public image.
FATAL:   While making image from oci registry: error fetching image to cache: failed to get checksum for docker://nfcore/scflow: Error reading manifest latest in docker.io/nfcore/scflow: manifest unknown: manifest unknown
```

I've also tried `nextflow run nf-core/scflow -r dev -profile singularity` wondering if it'll pull the container. 

Sorry, but I'm not familiar with Singularity, so the naive question. Could you please provide a minimal example of how to install the tool?

Thank you,
Susheel