fix(*): changed python to python3 in production dockerfiles

As mentioned here: https://github.com/docker-library/docker/issues/240 Alpine took python2 out of its distro, and our python install command no longer works on the production dockerfiles. Changing this to python3 seems to work – this is what Ketida does (and what was done in the CI pipeline).

That said: I am not sure what (if anything!) actually requires Python on production, and I don't know how to test this. Ben successfully deployed these changes to Kotahi Dev last night, so there's that?