Only build docker images for the master, staging and nightly branches.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

.gitlab-ci.yml

@@ -16,7 +16,7 @@ job_docker_master:
   only:
     - master
 
-job_docker_other:
+job_docker_nightly:
   stage: build
   image: docker:latest
   services:
@@ -27,8 +27,9 @@ job_docker_other:
     - docker build --pull -t "$CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG" -f docker/Dockerfile .
     - docker run --rm "$CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG" run-tests
     - docker push "$CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG"
-  except:
-    - master
+  only:
+    - nightly
+    - staging
 
 .test_base: &test_base
   stage: test