Error occurs when an image is used for a function rather than a handler

[hobgoblina]

I'm trying to deploy multiple functions, one of which needs to be deployed with an image attribute rather than a handler filepath. I'm getting this error for that function: TypeError: Cannot read properties of undefined (reading 'lastIndexOf') which occurs in getHandlerFilename

Not sure if there's a better way to handle this via the serverless config, but here's a PR that should fix it: #101