Docker image pull policy

We are currently basing our Dockerfile on the image tomcat:9-jdk8-openjdk-slim, which evolves over the time, it's basically a tomcat 9.x with openjdk8.

We have decided to use it because we want our environment to be up to date. The problem is that the current policy of our maven docker plugin won't update the image if we already have it in our docker registry.

The solution is to change that policy.