​You're correct, theoretically. But in reality is best to not stick a dozen services or apps in a single docker image. What's the point of using docker if you're having a single container with everything in it? Eg: something smells if you need to run a supervisor inside the container.

If we're talking about python packages managed by the os package manager ... there are very few situations where it makes sense to use them (eg: pysvn is especially hard to build), but other than that? Plus it's easier to cache some wheels than to cache some apt packages when building images, way easier.

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