Updating base docker image

[shimwell]

The newest image with python3.8 as default is currently used. However we could update to the newer miniconda image and create a python3.8 environment in the following way

FROM continuumio/miniconda3:4.10.3 as dependencies

# base image is python 3.9 but python 3.8 is needed for Cubit
# this creates and activates a new python 3.8 conda enviroment
RUN conda create --name py38 python=3.8
RUN echo "conda activate py38" >> ~/.bashrc
SHELL ["/bin/bash", "--login", "-c"]