Don't try and push the latest image on every build.
Don't try and push the latest image on every build.

* Publish latest tag on release.
* Publish master tag on master branch.

Signed-off-by: Ben Kochie <9bbf917dc63fbaa40c67921332d26cf16a50db75@gmail.com>

ncabatoff/process-exporter,ncabatoff/process-exporter