Update Dockerfile to use conda-forge compilers
Additionally I switched to the miniforge3 image as this bases its installation on conda-forge
and I also have used `mamba` to install the environment as this massively saves time.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
