feat: use centralized service images (gitlab.com)

mentioned in commit 2c3aba65

mentioned in merge request to-be-continuous/samples/python-on-kubernetes!44 (merged)

mentioned in merge request to-be-continuous/samples/php-cnb-on-kubernetes!41 (merged)

mentioned in merge request to-be-continuous/samples/golang-cnb-on-kubernetes!50 (merged)

mentioned in merge request to-be-continuous/samples/php-on-kubernetes!97 (merged)

mentioned in merge request to-be-continuous/samples/maven-on-kubernetes!101 (merged)