added explicit latest tag to docker image

.gitlab-ci.yml

@@ -4,7 +4,7 @@ stages:
 
 # Compile your project based on latest coq version.
 build_latest:
-  image: motrellin/docker-coq-equations
+  image: motrellin/docker-coq-equations:latest
   stage: build
   script:
     - make all