Update Dockerfile and image build in CI

This MR aims to do two things:

1. Fix the Docker image so that acorn is installed inside of the conda environment (see #107 (closed)).
2. Tag the latest commit on dev to acorn:latest. This will then allow us to have the image unpacked to cvmfs which will enable an easier quickstart for anyone with a cvmfs mounted machine

The unpacked image on cvmfs can then be run using singularity

singularity shell -e --nv --bind $PWD /cvmfs/unpacked.cern.ch/gitlab-registry.cern.ch/gnn4itkteam/acorn\:latest