readd docker login on deploy

This was removed during debugging and accidently forgotten to readd. It
fails to pass if no Docker credentials are given, which would be always
the case except on openwrtorg/master. In the future a test docker hub
account should be used instead of disabling it.

Fixes: 475f2f1a97

Signed-off-by: Paul Spooren <mail@aparcar.org>

.gitlab/ci/base.yml

@@ -23,3 +23,5 @@
     - master
   services:
     - docker:dind
+  before_script:
+    - docker login -u "$DOCKER_USER" -p "$DOCKER_PASS"