Trying to pass environment variable automatically

.gitlab-ci.yml

@@ -19,7 +19,7 @@ build-base:
     - docker info
     - echo "$CI_REGISTRY_PASSWORD" | docker login -u "$CI_REGISTRY_USER" --password-stdin
   script:
-    - docker build --pull -t "$CI_REGISTRY_IMAGE:$CI_COMMIT_SHORT_SHA" -t "$CI_REGISTRY_IMAGE:$GRAV_VERSION" .
+    - docker build --pull --build-arg "GRAV_VERSION=$GRAV_VERSION" -t "$CI_REGISTRY_IMAGE:$CI_COMMIT_SHORT_SHA" -t "$CI_REGISTRY_IMAGE:$GRAV_VERSION" .
     - docker push "$CI_REGISTRY_IMAGE:$GRAV_VERSION"
 
 build-git:

Dockerfile

@@ -1,6 +1,8 @@
 FROM nginx:1.15.10-alpine
 USER root
 
+ARG GRAV_VERSION
+
 # Install PHP And Modules Needed For Grav, With Optional Modules To Help With Performance
 RUN apk add --no-cache \
     php7 \