Improve CI configuration

Signed-off-by: Severin Kaderli <severin@kaderli.dev>

.woodpecker.yml

@@ -1,9 +1,11 @@
 pipeline:
   build-image:
-    image: docker
-    commands:
-      - cd "src/website"
-      - docker login -u "$REGISTRY_USER" -p "$REGISTRY_TOKEN" registry.gitlab.com
-      - docker build -t registry.gitlab.com/severinkaderli/registry:magsend-website .
-      - docker push registry.gitlab.com/severinkaderli/registry:magsend-website
-    secrets: [registry_user, registry_token]
+    image: woodpeckerci/plugin-docker-buildx
+    settings:
+      dockerfile: src/website/Dockerfile
+      registry: registry.gitlab.com
+      repo: severinkaderli/registry
+      username:
+        from_secret: registry_user
+      password:
+        from_secret: registry_token