el8: remove BASE_BRANCH CI variable

gitlab-ci doesn't support variables in patrigger parallel matrix arrays

.gitlab-ci.yml

-variables:
-  BASE_BRANCH: "el8"
-
 include:
   # configure build/scan/push jobs
   - component: git.ligo.org/computing/gitlab/components/docker/all@1
@@ -8,7 +5,7 @@ include:
       # path of main project
       upstream_project_path: docker/base
       # compare container_scanning results of new images to this one
-      default_image_name: "docker/base:${BASE_BRANCH}"
+      default_image_name: "docker/base:el8"
       # run these commands to test the image
       test_script:
         - dnf info lalsuite
@@ -31,14 +28,14 @@ docker_trigger_packaging:
   parallel:
     matrix:
       - BRANCH:
-          - ${BASE_BRANCH}-development
-          - ${BASE_BRANCH}-production
-          - ${BASE_BRANCH}-production-staging
-          - ${BASE_BRANCH}-resync
-          - ${BASE_BRANCH}-resync-staging
-          - ${BASE_BRANCH}-resync-testing
-          - ${BASE_BRANCH}-staging
-          - ${BASE_BRANCH}-testing
+          - el8-development
+          - el8-production
+          - el8-production-staging
+          - el8-resync
+          - el8-resync-staging
+          - el8-resync-testing
+          - el8-staging
+          - el8-testing
   trigger:
     branch: $BRANCH
     project: docker/packaging