Use container solution for covviz installation instead of conda

covviz is currently not available via bioconda and so we have been using pip to install it within conda environment. Current config:

name: covviz

channels:
  - conda-forge

dependencies:
  - python==3.8.5
  - pip
  - pip:
      - covviz==1.2.2

Using pip within conda env is not recommended at all though. When dealing with #49 (closed), it became clear that we needed to move away from this solution.

Since we just need covviz and no other tools need to be installed, we can sidestep conda env and instead we can directly use covviz docker container provided by the tool authors. Note that, we are already using similar setup to install and use somalier.