Run export-distribution from within a container

[fredvd]

I started up my project this afternoon from containers, tested a distribution install and then though I could update some content and export it again..... but I'd need to run export-distribution in the container. Using mostly the default plone-backend as built by cookiecutter-plone-starter scaffold

There are some caveats:

• export-distribution expects a zope.conf to be passed in, but the one generated in /app/etc/zope.conf needs interpolation from a lot of variables.
• /app/docker-entrypoint.sh does this as it sets all the var as environment variables before starting zop
• /app/docker-entrypoint.sh has a run command that also expects the zope.conf
• When I try to run /app/docker-entrypoint.sh run ./bin/export-distribution Plone when attaching to the container... I get an error that the data.fs is in use. This probably happening because I'm not using relstorage, so it is starting a local instance instead of zeoclient
• Lastly: trying to start the export-distribution script on container startup with the run command, either on the commandline of with a 'service' service' in the compose file, I see no output, no error either. But nothing is changed in the distribution folder. But the problem might be as well that that folder is not persisted anyway in a fs layer.

So... Do we actually want to support this and is the effort justified, or just put a big warning: only work on your distributions on 'from source' installed backends (for now).