fix: use build runner label for docker build workflows

Workflows need docker CLI which is only available in the docker:latest
container started by the 'build' runner label, not node:20-bookworm.

.gitea/workflows/rsyncd.yaml

@@ -11,7 +11,7 @@ env:
 
 jobs:
   build-and-push-image:
-    runs-on: docker
+    runs-on: build
     steps:
       - name: Checkout
         uses: actions/checkout@v5

.gitea/workflows/telegraf-snmp.yaml

@@ -11,7 +11,7 @@ env:
 
 jobs:
   build-and-push-image:
-    runs-on: docker
+    runs-on: build
     steps:
       - name: Checkout
         uses: actions/checkout@v5