fix(ci): lock slate version

.woodpecker/docs.yml

@@ -21,20 +21,17 @@ pipeline:
       - make api-docs
 
   slate-docs:
-    image: 'slatedocs/slate'
+    image: 'slatedocs/slate:v2.13.0'
     group: 'generate'
     # Slate requires a specific directory to run in
     commands:
-      - rm -rf /srv/slate/source
-      - mv "$CI_WORKSPACE/docs/api" /srv/slate/source
-      - cd /srv/slate
-      - /srv/slate/slate.sh build
-      - mv /srv/slate/build "$CI_WORKSPACE/docs/api-build"
+      - cd docs/api
+      - bundle exec middleman build --clean
 
   archive:
     image: 'alpine'
     commands:
-      - cp -r docs/api-build docs/public/api
+      - cp -r docs/api/build docs/public/api
       - 'cd docs/public && tar czvf ../../docs.tar.gz *'
       - 'cd ../../src/_docs && tar czvf ../../api-docs.tar.gz *'
     when: