build cobald image from slurm role, separated tags

roles/slurm/tasks/dockerimage.yml

@@ -1,8 +1,9 @@
 - name: build slurm base docker image
   include_tasks: dockerimage_build.yml
   vars:
+    slurm_image_prefix: "{{slurm_base_image_prefix | default('slurm') }}"
     image_name: base
-    dockerfile: slurm-base.Dockerfile
+    dockerfile: "{{lookup('file', 'slurm-base.Dockerfile')}}"
     files:
     - dest: entrypoint.sh
       content: "{{ lookup('file', 'entrypoint.sh') }}"
@@ -20,5 +21,5 @@
       "{{(slurm_baseimg_build_chg | default(False)) or
          slurm_baseimg_build.changed}}"
 
-- name: "build slurm base docker image {{name}}"
+- name: "build slurm base docker image {{image_name}}"
   include_tasks: dockerimage_build.yml