Add support for the --entrypoint option of docker run

Signed-off-by: Satoru Logic <satorulogic@gmail.com>
Order "fig run" options alphabetically

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Merge pull request #511 from bfirsh/support_entrypoint

Add support for entrypoint to "fig run"