Export is a way to dump the contents of your container into a .tar.gz, or a stream to put into some other place. For example, you could stream this into an in memory tar in python.
Usage
USAGE: singularity [...] export [export options...] <container path>
Export will dump a tar stream of the container image contents to standard
out (stdout).
EXPORT OPTIONS:
-f/--file Output to a file instead of a pipe
--command Replace the tar command (DEFAULT: 'tar cf - .')
EXAMPLES:
$ singularity export /tmp/Debian.img > /tmp/Debian.tar
$ singularity export /tmp/Debian.img | gzip -9 > /tmp/Debian.tar.gz
$ singularity export -f Debian.tar /tmp/Debian.img