run-local.sh is incorrect if image has never been built

[craig-willis]

With https://github.com/whole-tale/girder_wholetale/pull/558 the image is always in the manifest so the check when generating run-local.sh is no longer valid (https://github.com/whole-tale/girder_wholetale/blob/master/server/lib/exporters/bag.py#L218). We need to check if the image actually exists in the registry.

Steps to reproduce

• Create a new tale, but don't build it
• Export it
• Inspect run-local.sh

Expected results run-local.sh should contain command to build the image locally.

Actual results It doesn't.

[Xarthisius]

Or we could always output build_cmd saying that you can use it if docker run returns image not found.