maven.gitlab-ci.yml : update to use GitLab docker proxy

Update maven.gitlab-ci.yml to use GitLab docker proxy, docs here: https://docs.gitlab.com/user/packages/dependency_proxy/

This may be as simple as prefixing Docker image names referenced in the template with CI_DEPENDENCY_PROXY_GROUP_IMAGE_PREFIX

Followon issue from the AWS outage on 20/10/25 that is causing our pipeline to fail.