sylabs/singularity: Singularity 3.1.0 release candidate 4

Great news Singularity fans! Here comes a new feature release! And this is a big one! This marks the fourth release candidate of Singularity v3.1.0. Singularity v3.1.0 contains a ton of new features, enhancements and bug fixes. The big news is that there is now an OCI compliant variant of the Singularity runtime! (Thanks @cclerget!) Using the oci command group, a user can launch a fully OCI compliant version of Singularity. Here's a high-level overview of the changes in this release. For more details, see the git log. New Commands Introduced the oci command group to support a new OCI compliant variant of the Singularity runtime: attach Attach console to a running container process create Create a container from a bundle directory delete Delete container exec Execute a command within container kill Kill a container mount Mount create an OCI bundle from SIF image pause Suspends all processes inside the container resume Resumes all processes previously paused inside the container run Create/start/attach/delete a container from a bundle directory start Start container process state Query state of a container umount Umount delete bundle update Update container cgroups resources Added cache command group to inspect and manage cached files clean Clean your local Singularity cache list List your local Singularity cache New features / functionalities Can now build CLI on darwin for limited functionality on Mac Added the scratch bootstrap agent to build from anything Reintroduced support for zypper bootstrap agent Added the ability to overwrite a new singularity.conf when building from RPM if desired Fixed several regressions and omissions in SCIF support Added caching for containers pulled/built from the Container Library Changed keys command group to key (retained hidden keys command for backward compatibility) Created an RPMPREFIX variable to allow RPMs to be installed in custom locations Greatly expanded CI unit and end-to-end testing Please test this new release candidate in all of your exotic environments so that we can catch any bugs before they make it into GA. As always, please report any bugs to: https://github.com/sylabs/singularity/issues/new And if you think that you've discovered a security vulnerability please report it to: security@sylabs.io Thanks! Happy hacking