Define python version or virtualenv

[arokem]

Moving here from issue #285:

The original question was:

Is there a way to define which python version or command is used? e.g. launch with python3.8 instead of python? Also is there a way to tell it to use a virtualenv inside the image?

[arokem]

In general: you can use of a specific version of Python, by inheriting the docker image that is used to generate the cloudknot docker image from another image (see this example). You'll see that in code cell 4, we set a key-word argument base_image="python:3.7", which directs the image to be built inherting from the 3.7 tag of the official python image. Similarly, you can build your own docker image locally and build your CK image based on that image. This might help you from having to select a particular virtualenv inside the image.

If you are already using a base image that is not a python base image and therefore need to change the command python from python 2 to python 3 you can add the following to your docker image before doing any pip install:

# add python 3.8
RUN apt-get update && apt-get -y install software-properties-common
RUN add-apt-repository ppa:deadsnakes/ppa && \
    apt-get update && apt-get -y install python3.8 python3-pip python3.8-dev

RUN update-alternatives --install /usr/bin/python python /usr/bin/python3.8 1

#sort out pip
RUN cd /tmp && curl https://bootstrap.pypa.io/get-pip.py -o get-pip.py && python get-pip.py

(Thanks to @DrJohnDale for posting this on the issue)