ci: Do not build docker redundantly when tags are pushed

There are only two cases we care about docker builds:

- On branches (not tags): To verify that we didn't break the docker
  build.
- When a Github Release is published: To build & publish an official
  docker image.

In the second case, the build should happen after the Github Release is
published, NOT when a tag is pushed.

.github/workflows/docker.yml

@@ -3,6 +3,7 @@ name: Docker Image
 
 on:
   push:
+    tags-ignore: ['*']
     paths:
       - docker/**
       - .github/workflows/docker.yml