Docker build image workflow

[ianlewis]

Based on #23 it would be possible to allow the use of a generic Docker image to perform the build step prior to provenance generation.

Considerations:

• We could allow users to specify a publicly or privately available Docker image by name and run that.
• We could allow users to specify a Dockerfile which we build for them before running the image.

[rbehjati]

Can this be closed and marked as completed now that we have the container-based builder?

[asraa]

I think so - the second suggestion about provenance for a docker file is specified here: #57