feat(docker): .build-docker-image must accept kaniko arguments

Some users may requires to pass kaniko arguments to set, for example,
some build args required by the Dockerfile.

* templates/Docker.yaml: add `KANIKO_ARGS` optional variable
  overridable by users and pass it to executor command line.

templates/Docker.yaml

@@ -29,6 +29,7 @@
 #    - to `Dockerfile.${IMAGE_NAME}` if the file exists
 #    - to `Dockerfile` if the file exists
 # - `IMAGE_TAG`: defaults to `git-${CI_COMMIT_SHORT_SHA}`
+# - `KANIKO_ARGS`: arguments to pass to kaniko executor command, empty by default
 #
 # USED CI VARIABLES
 # =================
@@ -44,6 +45,8 @@
   image:
     name: gcr.io/kaniko-project/executor:v1.7.0-debug
     entrypoint: [""]
+  variables:
+    KANIKO_ARGS: ''
   script:
     - export IMAGE_NAME=${IMAGE_NAME:-${CI_JOB_NAME%-docker-build*}}
     - test -f Dockerfile.${IMAGE_NAME} && export DOCKERFILE=${DOCKERFILE:-Dockerfile.${IMAGE_NAME}} || true
@@ -51,7 +54,7 @@
     - export IMAGE_TAG=${IMAGE_TAG:-git-${CI_COMMIT_SHORT_SHA}}
     - mkdir -p /kaniko/.docker
     - echo "{\"auths\":{\"${CI_REGISTRY}\":{\"auth\":\"$(echo -n ${CI_REGISTRY_USER}:${CI_REGISTRY_PASSWORD} | base64 | tr -d '\n')\"}}}" > /kaniko/.docker/config.json
-    - /kaniko/executor --context ${CI_PROJECT_DIR} --dockerfile $CI_PROJECT_DIR/${DOCKERFILE} --destination ${CI_REGISTRY_IMAGE}/${IMAGE_NAME}:${IMAGE_TAG}
+    - /kaniko/executor ${KANIKO_ARGS} --context ${CI_PROJECT_DIR} --dockerfile $CI_PROJECT_DIR/${DOCKERFILE} --destination ${CI_REGISTRY_IMAGE}/${IMAGE_NAME}:${IMAGE_TAG}
 
 # Define `.tag-docker-image` template used by `*-docker-tag` jobs
 # It tag the `${IMAGE_NAME}:${SOURCE_TAG}` with the tag `${IMAGE_TAG}`