Better Docker/Mac/Linux Patterns

[metaskills]

Followup to #10 but where I learned that having a github gem source would cause issues. To solve this we have done the following.

• New lamby build contianer user for Linux interop.
• Done this in docker-compose build time. All compose runs will use.
• Avoid perm games on Darwin. Helps when using docker-sync.
• Move build cleanup back to _build script. When in Docker, adds TONS to image size.