Encapsulate manifold run.

[jelmersnoeck]

Manifold Run doesn't work on PRs due to Travis' set up with regards to
secrets.

Manifold Run is only needed when we release the image - to inject the
docker credentials. We don't need to do this when we run tests and
linting.

Let's move it to only run when we build the images (after merging a PR)
so that we can ensure PRs still build.

[codecov]

Codecov Report

Merging #126 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #126   +/-   ##
=======================================
  Coverage   33.69%   33.69%           
=======================================
  Files          26       26           
  Lines        2704     2704           
=======================================
  Hits          911      911           
  Misses       1718     1718           
  Partials       75       75

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9988c2b...3ace013. Read the comment docs.