cli: Add double dash to run usage
cli: Add double dash to run usage

This is so we can use flag-like arguments in the command without them
being parsed by docopt, e.g.:

flynn -a example run -e bash -- -c 'echo $FOO'

Signed-off-by: Lewis Marshall <748e1641a368164906d4a0c0e3965345453dcc93@lmars.net>

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