woodpecker: use buildah image for CI

.woodpecker/workflow.yaml

@@ -4,12 +4,12 @@ when:
 
 steps:
   - name: build
-    image: alpine
+    image: quay.io/buildah/stable
     commands:
-      - apk update
+      - buildah build keycloak/
-      - apk add buildah netavark
-      - buildah --storage-driver=vfs build keycloak/
     backend_options:
       kubernetes:
         securityContext:
+          runAsUser: 999
+          runAsGroup: 999
           privileged: true