Version pin postgres service

We guide the users to use `latest` which is dangerous since it can lead
to breaking changes for things we don't have control over. Specify the
latest version of postgres as an example, and show that the password
needs to be set from now on since postgres updated their image
configuration to always require the password.

reference https://gitlab.com/gitlab-org/gitlab-runner/issues/8323

doc/ci/services/postgres.md

@@ -17,12 +17,12 @@ First, in your `.gitlab-ci.yml` add:
 
 ```yaml
 services:
-  - postgres:latest
+  - postgres:12.2-alpine
 
 variables:
   POSTGRES_DB: nice_marmot
   POSTGRES_USER: runner
-  POSTGRES_PASSWORD: ""
+  POSTGRES_PASSWORD: "runner-password"
 ```
 
 NOTE: **Note:**
@@ -37,7 +37,7 @@ And then configure your application to use the database, for example:
 ```yaml
 Host: postgres
 User: runner
-Password:
+Password: runner-password
 Database: nice_marmot
 ```