In general you shouldn't need to "activate" a Conda environment in the shell. Things generally "just work" if you use absolute paths. Something like this:

  COPY conda.yaml /env/conda.yaml
  RUN conda env create -f /env/conda.yaml -p /env/conda

  COPY foo.py /env/foo.py
  CMD ["/env/conda/bin/python", "/env/foo.py"]

What is a little funny is installing a consistent version of Conda inside a container, because the official Miniconda installers are rolling-release only. However you might be able to downgrade to your desired version of Conda after installation.