Try to add testing repo.

.woodpecker.yml

@@ -5,6 +5,11 @@ clone:
       recursive: true
       submodule_update_remote: true
 pipeline:
+  check_formatting:
+    image: alpine:3
+    commands:
+      - apk add prettier --repository=https://dl-cdn.alpinelinux.org/alpine/edge/testing
+      - prettier -c src
   check_documentation_build:
     image: rust:1.61-slim-buster
     commands:
@@ -14,8 +19,3 @@ pipeline:
       - apt-get install git curl -y
       - ./update-includes.sh
       - mdbook build .
-  check_formatting:
-    image: alpine:3
-    commands:
-      - apk add prettier
-      - prettier -c src