[ci] Ensure we use docker-login before attempting to download containers from Idiap's registry

.gitlab-ci.yml

@@ -4,6 +4,7 @@
 # Definition of global variables (all stages)
 variables:
   CONDA_ROOT: "${CI_PROJECT_DIR}/miniconda"
+  DOCKER_REGISTRY: docker.idiap.ch
 
 
 # Definition of our build pipeline order
@@ -23,6 +24,7 @@ stages:
     - ./_ci/install.sh _ci master #installs ci support scripts
     - ./_ci/before_build.sh
     - docker info
+    - docker login -u gitlab-ci-token -p $CI_JOB_TOKEN docker.idiap.ch
     - ./scripts/before_test.sh build #pulls required docker images and compiles
   script:
     - ./_ci/build.sh