Fix version number

Version numbers for this project start with a `v`.

.gitlab-ci.yml

@@ -12,7 +12,7 @@ build-master:
     DOCKER_HOST: "tcp://docker:2375"
     DOH_VERSION: "2.0.1"
   script:
-    - docker build --pull --build-arg VERSION="$DOH_VERSION" -t "doh:testing" .
+    - docker build --pull --build-arg VERSION="v${DOH_VERSION}" -t "doh:testing" .
     - ./tools/tagging.sh "$CI_REGISTRY_IMAGE" "${DOH_VERSION:-latest}" ""
     - docker images --format "{{.Repository}}:{{.Tag}}" "$CI_REGISTRY_IMAGE" | xargs -L 1 docker push
   only: