services/horizon: Fix docker-compose Postgres auth

By not pinning the Postgres version, authentication stopped working since now it
requires either setting the Postgres password or an explicit
`POSTGRES_HOST_AUTH_METHOD=trust`:

```
Error: Database is uninitialized and superuser password is not specified.
       You must specify POSTGRES_PASSWORD for the superuser. Use
       "-e POSTGRES_PASSWORD=password" to set it in "docker run".

       You may also use POSTGRES_HOST_AUTH_METHOD=trust to allow all connections
       without a password. This is *not* recommended. See PostgreSQL
       documentation about "trust":
       https://www.postgresql.org/docs/current/auth-trust.html
```

Making this breaking change on a patch release is arguably a mistake on
Postgres' end (see
docker-library/postgres#681 (comment) )
but ... it is what it is. This commit sets `POSTGRES_HOST_AUTH_METHOD=trust` and
also pins the patch number of Postgres to make it future-proof.

Author: 2opremio

services/horizon/docker/docker-compose.yml

@@ -1,7 +1,7 @@
 version: '3'
 services:
   core-postgres:
-    image: postgres:9.6-alpine
+    image: postgres:9.6.17-alpine
     restart: on-failure
     environment:
       - POSTGRES_PASSWORD=mysecretpassword
@@ -30,9 +30,10 @@ services:
     network_mode: '${NETWORK_MODE:-bridge}'
 
   horizon-postgres:
-    image: postgres:9.6-alpine
+    image: postgres:9.6.17-alpine
     restart: on-failure
     environment:
+      - POSTGRES_HOST_AUTH_METHOD=trust
       - POSTGRES_DB=horizon
     ports:
       - "5432:5432"