feat(DockerBuilder): make ADD and COPY behaviour consistent with Docker

[nokome]

We support both ADD and COPY in the # dockter section of a Dockerfile

https://github.com/stencila/dockter/blob/v0.2.6/src/DockerBuilder.ts#L167

Docker's ADD and COPY instructions are very similar but COPY is generally recommended. Nonetheless, if we support ADD in the # dockter section of a Dockerfile then we should provide consistent behaviour.