Add build arg to kaniko image

.gitlab/ci_templates/kaniko.yml

@@ -8,6 +8,13 @@
 #
 # - KANIKO_CONTEXT: The subdir which holds the Dockerfile, leave unset if
 #                   the Dockerfile is located at root level of the project.
+#
+# - KANIKO_EXTEND_BASEIMAGE: Name of an image that is build in the same
+#   pipeline and is being used by your dockerfile as a baseimage.
+#   You need to specify the argument `baseimage` in your dockerfile and use
+#   this argument in the `FROM` instruction.
+#   see [how to use build arguments](https://docs.docker.com/engine/reference/builder/#arg)
+#   for more information.
 .kaniko_build:
   stage: build
   image:
@@ -18,4 +25,5 @@
   script:
     - echo "{\"auths\":{\"$CI_REGISTRY\":{\"username\":\"$CI_REGISTRY_USER\",\"password\":\"$CI_REGISTRY_PASSWORD\"}}}" > /kaniko/.docker/config.json
     - if [ -n "${KANIKO_BUILD_IMAGENAME}" ]; then  export IMAGENAME="/${KANIKO_BUILD_IMAGENAME}"; fi
-    - /kaniko/executor --context ${CI_PROJECT_DIR}/${KANIKO_CONTEXT:-.} --dockerfile ${CI_PROJECT_DIR}/${KANIKO_CONTEXT:-.}/Dockerfile --destination ${CI_REGISTRY_IMAGE}${IMAGENAME}:${CI_COMMIT_REF_NAME}
+    - if [ -n "${KANIKO_EXTEND_BASEIMAGE}" ]; then export BASEIMAGE="--build-arg baseimage=${CI_REGISTRY_IMAGE}/${KANIKO_EXTEND_BASEIMAGE}:${CI_COMMIT_REF_NAME}"; fi
+    - /kaniko/executor --context ${CI_PROJECT_DIR}/${KANIKO_CONTEXT:-.} --dockerfile ${CI_PROJECT_DIR}/${KANIKO_CONTEXT:-.}/Dockerfile --destination ${CI_REGISTRY_IMAGE}${IMAGENAME}:${CI_COMMIT_REF_NAME} ${BASEIMAGE}