Simplify Golang images

[briandealwis]

• Add GOTRACEBACK env var so skaffold debug can identify images as golang
• Use multi-stage builds to produce smaller images (as [per distroless[(https://github.com/GoogleContainerTools/distroless#examples-with-docker))

[sujit-kamireddy]

@briandealwis ,
Is having "ENV GOTRACEBACK=all" mandatory for Go Dockerfile?
Or are we adding it primarily for 'skaffold debug'?

My concern is that skaffold debug may not work for existing apps that doesn't have this env var. Should we fix skaffold debug instead to support such cases?

[briandealwis]

We need some way to identify the language runtime. There’s an open issue about defining an explicit mechanism (GoogleContainerTools/skaffold#2203), but this implicit environment approach is effective.