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The openjdk images are no longer updated, aside from early-access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

@@ -3,7 +3,7 @@ variables:
   GRADLE_OPTIONS: --info --stacktrace --warning-mode=all
 
 default:
-  image: openjdk:8-jdk-slim
+  image: eclipse-temurin:8
   cache:
     paths:
       - .gradle/caches