Update docker.yml

limit docker build to main branch

.github/workflows/docker.yml

@@ -3,7 +3,7 @@ name: Build docker image
 on:
   push:
     branches:
-      - '*'
+      - main
 
 jobs:
   build-image:
@@ -42,4 +42,4 @@ jobs:
           file: .github/docker/Dockerfile
           push: true
           tags: ${{ steps.meta.outputs.tags }}
-          labels: ${{ steps.meta.outputs.labels }}
+          labels: ${{ steps.meta.outputs.labels }}