Function code does not get bundled when a custom runtime image is defined

[julian-fh]

Specifying a custom runtime image for a function results in this error: ServerlessError: Unable to read handler file in function ....
This error also occurs if the file exists.

Example:
serverless.yml

functions:
  sentimentanalysis:
    image: immortalfaas/sentiment
    name: sentimentanalysis
    handler: handler.main

I suspect, that a recent change in the serverless framework causes this error.
The packaging gets interrupted, if a image is specified in a function.
This seems to be introduced to enable AWS Lambda image support.
https://github.com/serverless/serverless/blame/96735db3d4c08e18ca5c0b632fb0993e766a6894/lib/plugins/package/lib/packageService.js#L117

However, in serverless-openwhisk, this causes a failure of the packaging (and therefore the deployment).

A quick fix might be renaming image: to something like runtime_image: