Udocker

* Preliminary fix for of ELF paths in modes Fn for $ORIGIN:$ORIGIN
* Add Fn libraries for Ubuntu20, Fedora32, Fedora33
* Add Fn libraries for Alpine 3.12, 3.13

* Use hub.docker.com as default registry
* Search using v1 and v2 APIs
* Implement API /v2/search/repositories
* Adjust search results to screen size
* List container size with ps -s
* List container execution modes with ps -m
* Added support for nameat() and statx() in Pn and Fn modes
* Added Fn libraries for Ubuntu18, Ubuntu19, Fedora29, Fedora30, Fedora31, CentOS8
* Added Fn libraries for Alpine 3.8, 3.9, 3.10, 3.11
* Added support for sha512 hashes
* Added support for opaque whiteouts
* Added search --list-tags to available tags for a given repository
* Add CLI support for image names in format host/repository:tag
* Support for fake root in Sn execution modes via --user=root
* Improve verify of loaded/pulled images
* Improve handling of mountpoints
* Added --containerauth to enable direct use of the container passwd and group
* Added support for file mount bindings in singularity
* Added `UDOCKER_USE_PROOT_EXECUTABLE` env var to select proot location
* Added `UDOCKER_USE_RUNC_EXECUTABLE` env var to select runc location
* Added `UDOCKER_USE_SINGULARITY_EXECUTABLE` env var to select singularity
* Added `UDOCKER_DEFAULT_EXECUTION_MODE` env var to select default execution mode
* Added R2 and R3 execution modes for PRoot overlay execution in runc
* Added setup --purge for cleanup of mountpoints and files
* Added setup --fixperms to fix container file permissions
* Added run --env-file= to load file with environment variables
* Improve file and directory binding support for Singularity and runc
* Add command rename for renaming of containers
* Create processes without shell context
* Safer parsing of config files and removal of directories
* Improve installation
* Improved fix of SECCOMP accelerated mode for P1 mode
* Added loading and handling of container images in OCI format
* Fixes for udocker in ARM aarch64
* Fix processing of --dri in Sn mode: closes 241
* Improve handling of container and host authentication: addresses 239
* Fixes to address authentication and redirects in pull: closes 225, 230
* Added minimal support to load OCI images: closes 111
* Added Pn support for newer distributions: closes 192
* Improve the installation of udockertools: closes 220, 228
* Add --env-file= - to read environment variables from file: closes 212
* Prepare for pypy: closes 211
* Fixes for verification of container images: closes 209
* Fix command line processing for "-" in argument: closes 202
* Fix file protections on extraction making files u+r : closes 202, 206
* Fix comparison of kernel versions having non-integers: closes 183
* Support for both manifest V2 schema 1 and schema 2: closes 218, 225
* Further improved pathname translation in Fn modes: closes 160
* Implement save images in docker format: closes 74
* useradd and groupadd not working in containers: closes 141
* fix return code when exporting to stdin: closes 202

* Support for nvidia drivers on ubuntu: closes 162
* Installation improvements: closes 166
* Fix issue on Fn mode symlink conversion: addresses 160

* Improve parsing of quotes in the command line: closes 98
* Fix version command to exit with 0: closes 107
* Add kill-on-exit to proot on Pn modes
* Improve download of udocker utils
* Handle authentication headers when pulling: closes 110
* Handle of redirects when pulling
* Fix registries table
* Support search quay.io
* Fix auth header when no standard Docker registry is used
* Add registry detection on image name
* Add --version option
* Force python2 as interpreter: closes 131
* Fix handling of volumes in metadata
* Handle empty metadata
* Fix http proxy functionality: closes 115
* Ignore --no-trunc and --all in the images command: closes 108
* Implement verification of layers in manifest
* Add --nvidia to support GPUs and related drivers
* Send download messages to stderr
* Enable override of curl executable
* Fix building on CentOS 6: closes 157
* Mitigation for upstream limitation in runC without tty: closes 132
* Fix detection of executable with symlinks in container: closes 118
* Updated runC to v1.0.0-rc5
* Experimental support for Alpine in Fn modes
* Improve pathname translation in Fn modes for mounted dirs: addresses 160

* New execution engine using singularity
* Updated documentation with OpenMPI information and examples
* Additional unit tests
* Redirect messages to stderr
* Improved parsing of quotes in the command line: closes 87
* Allow override of the HOME environment variable
* Allow override of libfakechroot.so at the container level
* Automatic selection of libfakechroot.so from container info
* Improve automatic install
* Enable resetting prefix paths in Fn modes in remote hosts
* Do not set `AF_UNIX_PATH` in Fn modes when the host /tmp is a volume
* Export containers in both docker and udocker format
* Import containers docker and udocker format
* Load, import and export to/from stdin/stdout
* Clone existing containers
* Support for TCP/IP port remap in execution modes Pn
* Fix run with basenames failing: closes 89
* Allow run as root flag: closes 91

* Support image names prefixed by registry similarly to docker
* Add execution engine selection logic
* Add fr execution engine based on shared library interception
* Add rc execution engine based on rootless namespaces
* Improve proot tmp files cleanup on non ext filesystems
* Improve search returning empty on Docker repositories
* Improve runC execution portability
* Add environment variable `UDOCKER_KEYSTORE`: closes 75
* Prevent creation of .udocker when `UDOCKER_KEYSTORE` is used: closes 75