Split up CI into multiple jobs

[cmacmackin]

It would be good to have separate CI jobs for building Platypus, building the documentation, building/running unit test, and running integration tests. (Obviously the test ones would need to be dependent on the build one and have access to the build artifacts). That way, even if one of the sets of tests fails it can still run the others and report on any issues that may in them as well.

This will require reworking how the build is currently done, e.g. by only building Platypus when building the image and then having separate jobs that run within containers using that image.