add instructions to convert Dockerfile to singularity

[tavareshugo]

If starting for a Dockerfile (rather than an image), the process is a bit more laborious:

• Need to have Docker installed
• First create a docker image: docker build -t my-docker-image:latest .
• Then convert it to singularity image: singularity build my-singularity-image.sif docker-daemon://my-docker-image:latest

[tavareshugo]

This could be a good example:

https://github.com/ComparativeGenomicsToolkit/Comparative-Annotation-Toolkit?tab=readme-ov-file