Switch CI from openjdk to eclipse-temurin images

The openjdk images are no longer updated, aside from early-access
builds. See:

* https://hub.docker.com/_/openjdk/
* https://github.com/docker-library/openjdk/issues/505

.gitlab-ci.yml

 build:
   stage: build
-  image: openjdk:8-jdk-slim
+  image: eclipse-temurin:8
   variables:
     GRADLE_USER_HOME: "$CI_PROJECT_DIR/.gradle"
   before_script: