Be more flexible with Docker image selection
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
Reviewed By: dereckson
Subscribers: Nasqueron Docker deployment squad
Tags: Docker images
Differential Revision: https://devcentral.nasqueron.org/D2369