build the container image locally using a slightly improved
container definition. it is know pulling a debian image and
it deletes the dorado tar ball before generating the .sif.

TODO: this definition must freeze versions of all dependencies
to ensure reproducibility and compatibility over time.

this commit also deletes old definitions and moves the
previous 'singultarity_containers' to just 'containers'.

lastly, the nextflow.config has been altered to use the local
container image instead of pulling it from the cloud.