CKAN is vulnerable to session secret shared across instances using Docker images

CKAN is an open-source DMS (data management system) for powering data hubs and data portals. When creating a new container based on one of the Docker images listed below, the same secret key was being used by default. If the users didn't set a custom value via environment variables in the `.env` file, that key was shared across different CKAN instances, making it easy to forge authentication requests. Users overriding the default secret key in their own `.env` file are not affected by this issue. Note that the legacy images (ckan/ckan) located in the main CKAN repo are not affected by this issue. The affected images are ckan/ckan-docker, (ckan/ckan-base images), okfn/docker-ckan (openknowledge/ckan-base and openknowledge/ckan-dev images) keitaroinc/docker-ckan (keitaro/ckan images).