POSTGRES_VERSION has no effect in test stage of Auto DevOps

Summary

POSTGRES_VERSION is not controlling the postgres container version like it is documented here https://docs.gitlab.com/ee/topics/autodevops/#environment-variables

Steps to reproduce

Enable Auto Devops, create a npm template project and set POSTGRES_VERSION = 9.6.12 and POSTGRES_ENABLED = true.

What is the current bug behavior?

Watch the logs in the test step and you'll see

Running with gitlab-runner 11.9.0-rc2 (227934c0)
  on docker-auto-scale fa6cab46
Using Docker executor with image gliderlabs/herokuish:latest ...
Starting service postgres:latest ...
Pulling docker image postgres:latest ...
Using docker image sha256:9c116111eb0806d217a0ce565dfbf4e2ed0dcde13976a7ed61e5ba60b269a5a8 for postgres:latest ...
Waiting for services to be up and running...

At the top of the job logs.

What is the expected correct behavior?

Watch the logs in the test step and expect

Running with gitlab-runner 11.9.0-rc2 (227934c0)
  on docker-auto-scale fa6cab46
Using Docker executor with image gliderlabs/herokuish:latest ...
Starting service postgres:$POSTGRES_VERSION ...
Pulling docker image postgres:$POSTGRES_VERSION ...
Using docker image sha256:9c116111eb0806d217a0ce565dfbf4e2ed0dcde13976a7ed61e5ba60b269a5a8 for postgres:$POSTGRES_VERSION ...
Waiting for services to be up and running...

At the top of the job logs.

Relevant logs and/or screenshots

Running with gitlab-runner 11.9.0-rc2 (227934c0)
  on docker-auto-scale fa6cab46
Using Docker executor with image gliderlabs/herokuish:latest ...
Starting service postgres:latest ...
Pulling docker image postgres:latest ...
Using docker image sha256:9c116111eb0806d217a0ce565dfbf4e2ed0dcde13976a7ed61e5ba60b269a5a8 for postgres:latest ...
Waiting for services to be up and running...

Output of checks

This bug happens on GitLab.com