An easier solution to the above issue is to use multi-stage docker containers where you can copy the content from one to another. In the above case you can have openjdk:slim as the base container and then use content from a python container to be copied over into this base container as follows:
FROM openjdk:slim
COPY --from=python:3.6 / /
...
<normal instructions for python container continues>
...
This feature is available as of Docker 17.05 and there are more things you can do using multi-stage build as in copying only the content you need from one to another.
Reference documentation