Udocker-stimela

* Support for nvidia drivers on ubuntu
- closes: 162
* Installation improvements
- closes: 166
* Fix issue on Fn mode symlink convertion
- partially 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
- partially 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

* Documentation fixes

* Support for import Docker containers in newer metadata structure
* Improve the command line parsing
* Improve temporary file handling and removal
* Support for additional execution engines to be provided in the future
* Improved parsing of entrypoint and cmd metadata
- closes: 53
* Increase name alias length
- closes: 52
* Add support for change dir into volume directories
- closes: 51
* Fix deletion of files upon container import
- closes: 50
* Fix exporting of host environment variables to the containers
- closes: 48
* Change misleading behavior of import tarball from move to copy
- closes: 44
* Fix validation of volumes specification
- closes: 43