docs: Add upgrade instructions for old postgres container.
docs: Add upgrade instructions for old postgres container.

Fixes #123

Signed-off-by: Alexander Trost <88b6b84d9fa0dbf878e34ae3d402bc4895fcd37e@googlemail.com>

Galexrt/docker-zulip,Galexrt/docker-zulip,Galexrt/docker-zulip