Adds a docker-login command to the bootstrap of builds requiring it

This MR tries to fix beat/beat.nightlies#1 by issuing a docker login in builds that need access to the internal docker registry.

bob/devtools/data/gitlab-ci/beat-docker-package.yaml

 build_linux_36:
+  extends:
+    - .build_linux_template
+    - .docker_bootstrap
   tags:
     - docker-build
 
 build_linux_37:
+  extends:
+    - .build_linux_template
+    - .docker_bootstrap
   tags:
     - docker-build
+
+build_macosx_36:
+  extends:
+    - .build_macosx_template
+    - .docker_bootstrap
+
+build_macosx_37:
+  extends:
+    - .build_macosx_template
+    - .docker_bootstrap