CI: simplify image build job

.gitlab-ci/build.yml

@@ -1,5 +1,5 @@
 ---
-.build-container:
+pipeline-image:
   cache:
     key: $CI_COMMIT_REF_SLUG
     paths:
@@ -11,22 +11,14 @@
     name: gcr.io/kaniko-project/executor:debug
     entrypoint: ['']
   variables:
-    TAG: $CI_COMMIT_SHORT_SHA
-    PROJECT_DIR: $CI_PROJECT_DIR
-    DOCKERFILE: Dockerfile
     GODEBUG: "http2client=0"
   script:
     - echo "{\"auths\":{\"$CI_REGISTRY\":{\"auth\":\"$(echo -n ${CI_REGISTRY_USER}:${CI_REGISTRY_PASSWORD} | base64)\"}}}" > /kaniko/.docker/config.json
     - /kaniko/executor --cache=true
                        --cache-dir=image-cache
-                       --context $PROJECT_DIR
-                       --dockerfile $PROJECT_DIR/$DOCKERFILE
+                       --context $CI_PROJECT_DIR
+                       --dockerfile $CI_PROJECT_DIR/pipeline.Dockerfile
                        --label 'git-branch'=$CI_COMMIT_REF_SLUG
                        --label 'git-tag=$CI_COMMIT_TAG'
                        --destination $PIPELINE_IMAGE
                        --log-timestamp=true
-
-pipeline-image:
-  extends: .build-container
-  variables:
-    DOCKERFILE: pipeline.Dockerfile