Cybercarpentry workshop On Containers
University of Arizona Singularity Tutorials
Dolmades - Windows Apps in Linux Docker-Singularity Containers Warning not tested
Gregory Kurtzer, creator of Singularity has provided two good talks online: Introduction to Singularity, and Advanced Singularity.
Vanessa Sochat, lead developer of Singularity Hub, also has given a great talk on Singularity which you can see online.