Added docs for postgres

container/postgres/README.md

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+This is a more complex librarian setup, and so requires a little more thought when setting
+it up.
+
+A few things to keep in mind when deploying the librarian:
+
+- This is inherently not secure by default; you will need to change the administrator
+  password for the librarian and potentially provision further accounts.
+
+- You will need to mark both the hostname of the docker container and of your
+  local machine as available for local transfers.
+
+- You will need to make sure that the local store locations are mounted
+  transparently by the docker container (i.e. you should have /path/to/store
+  the same inside and outside of the container).
+
+Specifically for this librarian, as we have the database running in a different Docker
+container, we'll need to make sure that this is up and running before running anything
+like a database migration.
+
+So, you will need to:
+
+- `bash pre_docker_setup.sh`
+- `docker compose build`
+- `docker compose up` (this will cause errors!)
+- `docker ps` -> grab the ID of the running librarian server instance
+- `docker exec -ti {ID} bash` -> into the container
+- `source setup_vars.sh`
+- `librarian-server-setup`
+- `exit`
+- Restart the docker container.
+- Your application should now be happily running.