Merge branch 'docker-release-only' into 'develop'

GitLab CI: only run Docker job when a tag is pushed

See merge request soapbox-pub/soapbox!2299

.gitlab-ci.yml

@@ -157,11 +157,11 @@ docker:
   # https://medium.com/devops-with-valentine/how-to-build-a-docker-image-and-push-it-to-the-gitlab-container-registry-from-a-gitlab-ci-pipeline-acac0d1f26df
   script:
     - echo $CI_REGISTRY_PASSWORD | docker login -u $CI_REGISTRY_USER $CI_REGISTRY --password-stdin
-    - docker build -t $CI_REGISTRY_IMAGE .
-    - docker push $CI_REGISTRY_IMAGE
-  only:
-    variables:
-      - $CI_DEFAULT_BRANCH == $CI_COMMIT_REF_NAME
+    - docker build -t $CI_REGISTRY_IMAGE:$CI_COMMIT_TAG .
+    - docker push $CI_REGISTRY_IMAGE:$CI_COMMIT_TAG
+  rules:
+    - if: $CI_COMMIT_TAG
+  interruptible: false
 
 release:
   stage: release