Build a docker image to use for testing in Github Actions

[gonuke]

You have successfully made a dockerfile that generates a docker image that can be used locally for testing.

This step is to make a dockerfile that generates a docker image that installs all the dependencies and stops short of actually running the tests. Then you push this docker image to the Github Container Registry and then use that docker image, instead of ubuntu-latest to run your tests, and no longer need to install numpy, pytest, coverage.

1. write a new Dockerfile
2. write a new action that will build and push this docker image when the Dockerfile changes
3. change existing action to use the new docker image

[gonuke]

Closed by #9