use minimal base image for lambda example

[imjasonh]

It turns out the base image doesn't matter, so long as you invoke the handler using the aws-lambda-go handler wrapper

https://docs.aws.amazon.com/lambda/latest/dg/go-image.html#go-image-other describes using alpine as a base, but ko's default cgr.dev/chainguard/static works just as well.

This brings the image size from 112MB to 6MB 📉 (zero vulns either way)

[mattmoor]

Nice 😁