Use mamba for conda env dependency install in Dockerfile

[srstsavage]

Installing dependencies using conda hangs for a very long time now (I gave up after 30+ minutes). Using mamba instead reduces the dependency install time to ~392 seconds in my testing. Still not great, but workable.

[joefutrelle]

Thanks, looks like a good enhancement.

In the near future we'll be pushing images to Docker Hub alleviating the need for end-users to run this messy and time-consuming build process.