Push new versions to DockerHub #21
Description
Some services, whose source code is on GitHub, use jupyter-math as a base. Every time a new version of jupyeter-math is released we should make sure that is pushed to DockerHub and re-used by other open-source services.
Note: we'll also continue to automatically push the images with the current workflow.
### Tasks
- [ ] Add a CI action that pushes to DockerHub
fyi, @GitHK, @pcrespov, @JavierGOrdonnez. I will take care of adding this to the CI.