feat: Only build image on tag push

Idea is that tag pushes mark what kind of release we are dealing with.

.github/workflows/build.yaml

@@ -1,7 +1,7 @@
 name: Create and publish a Docker image
 on:
   push:
-    branches: ["master"]
+    tags: ["*"]
 
 env:
   REGISTRY: ghcr.io