Potentially simplified user instructions

Here's an alternate set of instructions that may be easier - it eliminates the step of building the image, but it makes the command-line longer. The Docker registry in GitLab has been enabled to store images of this project. Might be good to try both process and see which would work best for uses, and then either merge or cancel this request depending.

This changes the process so that the user does not need to build an image prior to using Caption Whale. Instead, the image is downloaded from GitLab docker registry on the first run.

This also fixes the anti-pattern of allowing secrets to reside in image files -- a side-effect of the current instructions.

Advantages:

• Fewer setup steps
• Reduces the possibility that a secret will escape from the user's computer

Disadvantage:

• Longer command line in the docker run step