Automated build and push to cloud.sylabs.io on success

[bcdarwin]

Currently the container takes a while to build (and might fail e.g. due to various non-pinned dependencies) making it especially painful to do manually.

I think cloud.sylabs.io has automated builders and a free tier with a certain limit on monthly build minutes which would be the obvious first thing to try. [Note that this cloud only has space for one image at a time so we'd need to delete the previous image or find a way to upgrade our disk limits.]