Use CUSTREL for the custom release string

The RELEASE string is used commonly by multiple parties
We set a custom CUSTERL variable, for this special case, to push in the
special string "bullseyebootstrap"

.gitlab-ci.yml

@@ -7,6 +7,7 @@ stages:
 
 variables:
   RELEASE: "bullseye"
+  CUSTREL: "bullseyebootstrap"
   DISTRO: debian
   MIRROR: https://deb.debian.org/debian/
   STABLE: auto
@@ -74,13 +75,13 @@ build-base-rootfs:
       /kaniko/executor
       --context $CI_PROJECT_DIR/$image
       --dockerfile $CI_PROJECT_DIR/$image/Dockerfile
-      --destination $CI_REGISTRY_IMAGE/$RELEASE-$image:$TEST_TAG
+      --destination $CI_REGISTRY_IMAGE/$CUSTREL-$image:$TEST_TAG
       --build-arg OSNAME=$DISTRO
-      --build-arg RELEASE=$RELEASE
+      --build-arg RELEASE=$CUSTREL
       --build-arg TAG=$TEST_TAG
       --build-arg REGISTRY=$CI_REGISTRY_IMAGE
       --single-snapshot
-    - echo Pushed $CI_REGISTRY_IMAGE/$RELEASE-$image:$TEST_TAG.
+    - echo Pushed $CI_REGISTRY_IMAGE/$CUSTREL-$image:$TEST_TAG.
 
 .publish-docker-image:
   image: buildah/buildah
@@ -88,7 +89,7 @@ build-base-rootfs:
     - lightweight
   variables:
     AUTH: $CI_REGISTRY_USER:$CI_REGISTRY_PASSWORD
-    IMAGE: ${CI_REGISTRY_IMAGE}/${RELEASE}-${image}
+    IMAGE: ${CI_REGISTRY_IMAGE}/${CUSTREL}-${image}
   script:
     - echo "Tagging ${TEST_TAG} as ${TAG} for ${IMAGE}"
     - skopeo copy --src-creds "$AUTH" --dest-creds "$AUTH" docker://"${IMAGE}:${TEST_TAG}" docker://"${IMAGE}:${TAG}"
@@ -118,7 +119,7 @@ test-package-source-builder:
   needs:
     - build-package-source-builder-docker-image
   image:
-    name: $CI_REGISTRY_IMAGE/${RELEASE}-package-source-builder:${TEST_TAG}
+    name: $CI_REGISTRY_IMAGE/${CUSTREL}-package-source-builder:${TEST_TAG}
     entrypoint: [ "" ]
   script:
     - git clone https://gitlab.apertis.org/infrastructure/test-data/fixture-dash-non-updated