Fixing golang Dockerfiles to work with skaffold debug

[j-windsor]

Fixes #556.

Change summary:

• Allow skaffold to pass in go build flags
• Set environment variable skaffold looks for to detect go debug environment
• cleanup of the workdir for consistency in pathmapping.

Remaining issues / concerns:

[kelsk]

@GoogleCloudPlatform/anthos-dpe PTAL - These changes help enable debugging the microservices demo using skaffold, Cloud Code, and Cloud Shell Editor.

[askmeegs]

Tested locally; LGTM;