Troubleshoot DADA2 image in Singularity

Including a reference to any version of `docker://blekhmanlab/dada2` as the container for a singularity rule results in an impossibly long processing time for whatever kind of conversion is happening behind the scenes. We need to fix it, or at the very least bite the bullet and let it take however long to build a .sif file for a more recent version, so we can use that in the pipeline. The main suspect is whatever reference files are causing the container image (or at least the .sif version) to blow up to 1.3 GB.