[icebraining]

You could keep Postgres in an immutable image, with only /var/lib/postgres in a separate volume. Upgrading the PG config would just be a matter of unmounting it, replacing the image and re-mounting. (Docker automates this with its "data volumes", but you can do it manually too).

[wstrange]

In theory yes - but that strategy doesn't always work. Sometimes the implementation of the data store changes between releases - requiring an upgrade or data migration.

For large datasets that can take hours / days.

[icebraining]

That may prevent the simple unmount/replace image/mount workflow, but it doesn't prevent the separation between the mutable and immutable parts of the DBMS.