Fix or remove dockerfile

[lukethehuman]

The sledo dockerfile was used in the CI to build sledo on top of a base image containing MOOSE. As of version 0.2.0, sledo no longer requires MOOSE directly so this docker container may not be required. However, a working docker container may have value for the portability of sledo. Update the dockerfile such that it works for the latest versions of sledo and MOOSE, else remove it from the repository.