Deploy multiple versions of docker image, -scratch, -alpine, etc.

Right now, it's hard to add the grpcurl docker image to a kubernetes cluster to be able to debug other services. If, instead of building just one image that uses `from scratch` (related issue: https://github.com/fullstorydev/grpcurl/issues/235 ), there was some kind of -scratch / -alpine variant (with default still being scratch), then it would be far easier to attach and debug using grpcurl.

Contributions welcome? ^-^

Start by reviewing how the grpcurl Docker image is currently built and read related issue #235 for the existing scratch-image context. Define image variants such as the default scratch image and an Alpine-based alternative, then verify that each can be selected for Kubernetes debugging while preserving the current default.

Written by the indexing model from the issue text.