Switch from mamba to conda to preserve environment.yaml env vars (#44)

Author: jsignell

Dockerfile

@@ -6,7 +6,7 @@ USER ${NB_USER}
 
 ADD environment.yml environment.yml
 
-RUN mamba env update --prefix /srv/conda/envs/notebook --file environment.yml
+RUN conda env update --prefix /srv/conda/envs/notebook --file environment.yml
 
 COPY --chown=${NB_USER}:${NB_USER} image-tests /srv/repo/image-tests
 COPY --chown=${NB_USER}:${NB_USER} scripts /srv/repo/scripts