Be more flexible with Docker image selection

Summary:
Plan is to add image update feature, with user container rebuild.
That will write nasqueron/arcanist in enough place to create a
constant to ease forks or custom tags.

Test Plan: run arc wrapper

Reviewers: dereckson

Reviewed By: dereckson

Subscribers: Nasqueron Docker deployment squad

Tags: Docker images

Differential Revision: https://devcentral.nasqueron.org/D2369