docs: Add upgrade instructions for old postgres container.

Fixes #123

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

README.md

@@ -316,6 +316,12 @@ the first time, it will run the necessary database migrations with
 If you ever find you need to downgrade your Zulip server, you'll need
 to use `manage.py migrate` to downgrade the database schema manually.
 
+If you are using old `galexrt/docker-zulip` images (from Zulip 1.8.1 or
+older), you need to upgrade the postgres image from
+`quay.io/galexrt/postgres-zulip-tsearchextras:latest`.  Refer to the
+[instructions for upgrading from the old galexrt/docker-zulip](#upgrading-from-the-old-galexrtdocker-zulip)
+section.
+
 ### Using `docker-compose`
 
 0. (Optional) Upgrading does not delete your data, but it's generally
@@ -379,6 +385,73 @@ and the commit to use as `ZULIP_GIT_URL` and `ZULIP_GIT_REF`
 
 Then stop and restart the container as described in the previous section.
 
+### Upgrading from the old galexrt/docker-zulip
+
+If you are using an earlier version of `galexrt/docker-zulip` which
+used the `quay.io/galexrt/postgres-zulip-tsearchextras:latest`
+Postgres image, you need to run a few manual steps to upgrade to the
+`zulip/zulip-postgresql` Postgres image (because we've significantly
+upgraded the major postgres version).
+
+These instructions assume that you have not changed the default
+Postgres data path (`/opt/docker/zulip/postgresql/data`) in your
+`docker-compose.yml`.  If you have changed it, please replace all
+occurences of `/opt/docker/zulip/postgresql/data` with your path.
+
+1. Make a backup of your Zulip Postgres data dir.
+
+2. Stop all Zulip containers, except the postgres one (e.g. use
+   `docker stop` and not `docker-compose stop`).
+
+3. Create a new (upgraded) Postgres container using a different data directory:
+```
+docker run -d \
+      --name postgresnew \
+      -e POSTGRES_DB=zulip \
+      -e POSTGRES_USER=zulip \
+      -e POSTGRES_PASSWORD=zulip \
+      -v /opt/docker/zulip/postgresql/new:/var/lib/postgresql/data:rw \
+      zulip/zulip-postgresql:latest
+```
+
+4. Use `pg_dumpall` to dump all data from the existing Postgres container to
+the new Postgres container (replace `ZULIP_DATABASE_CONTAINER_NAME` with the
+name of the old Postgres container):
+```
+docker exec \
+    ZULIP_DATABASE_CONTAINER_NAME pg_dumpall -U postgres | \
+    docker exec -i postgresnew psql -U postgres
+```
+
+5. Stop and remove both Postgres containers:
+```
+docker stop ZULIP_DATABASE_CONTAINER_NAME postgresnew
+docker rm ZULIP_DATABASE_CONTAINER_NAME postgresnew
+```
+
+6. Edit your `docker-compose.yml` to use the
+`zulip/zulip-postgresql:latest` image for the `database` container
+(this is the default in `zulip/docker-zulip`).
+
+7. Replace the old Postgres data directory with upgraded data directory:
+```
+mv /opt/docker/zulip/postgresql/data /opt/docker/zulip/postgresql/old
+mv /opt/docker/zulip/postgresql/new /opt/docker/zulip/postgresql/data
+```
+
+8. Delete the old existing containers:
+```
+docker-compose rm
+```
+
+9. Start Zulip up again:
+```
+docker-compose up
+```
+
+That should be it. Your Postgres data has now been updated to use the
+`zulip/zulip-postgresql` image.
+
 ## Troubleshooting
 
 Common issues include: