fix(ci): correct image for generating slate docs

.woodpecker/docs.yml

@@ -26,9 +26,9 @@ pipeline:
     # Slate requires a specific directory to run in
     directory: '/srv/slate'
     commands:
-      - mv "$CI_WORKSPACE/docs/api" source
+      - mv "${CI_WORKSPACE}/docs/api" source
       - bundle exec middleman build --clean --watcher-disable
-      - mv build "$CI_WORKSPACE/docs/api-build"
+      - mv build "${CI_WORKSPACE}/docs/api-build"
 
   archive:
     image: 'alpine'