Unmerge docker-build and docker-push jobs

[etpinard]

Commit 1b78585 merged the docker-build and docker-push CircleCI jobs into a single docker-build-and-push as a workaround for intermittent CI failures involving _ "missing image"_ error duiring the docker-push step. See #63 (comment)

At present, this increases our total test time by about 2 minutes. This gap is likely to increase as we add more and more image-exporter tests. Therefore, we should try at some point to unmerge those two jobs.

[etpinard]

Not worth pursuing.