ci: Run Docker workflow on all branches

Tags are explicitly ignored on push. We still want branches to be
processed, but they won't if the `branches` property is missing. See
more detail here:

https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#onpushbranchestagsbranches-ignoretags-ignore

.github/workflows/docker.yml

@@ -3,7 +3,11 @@ name: Docker Image
 
 on:
   push:
+    # Tags are explicitly ignored on push. We still want branches to be processed, but they won't if
+    # the `branches` property is missing. See more detail here:
+    # https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#onpushbranchestagsbranches-ignoretags-ignore
     tags-ignore: ['*']
+    branches: ['*']
     paths:
       - docker/**
       - .github/workflows/docker.yml