fix(ci): Add auth debug step, switch back to buildx

Added a docker-auth-test step that:
- Shows credential lengths (for debugging)
- Tests docker login directly with Harbor

This will help identify if the issue is with secrets injection
or with how buildx handles authentication.

Reverted to woodpeckerci/plugin-docker-buildx since plugins/docker
requires server-side WOODPECKER_PLUGINS_PRIVILEGED config.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

.woodpecker.yml

@@ -84,13 +84,36 @@ steps:
   # ======================
   # Requires secrets: harbor_username, harbor_password
 
+  # Debug step - test registry auth before building
+  docker-auth-test:
+    image: docker:cli
+    environment:
+      HARBOR_USER:
+        from_secret: harbor_username
+      HARBOR_PASS:
+        from_secret: harbor_password
+    commands:
+      - echo "Testing Harbor authentication..."
+      - echo "Username length:" $(echo -n "$HARBOR_USER" | wc -c)
+      - echo "Password length:" $(echo -n "$HARBOR_PASS" | wc -c)
+      - echo "$HARBOR_PASS" | docker login reg.mosaicstack.dev -u "$HARBOR_USER" --password-stdin
+      - echo "Login successful!"
+      - docker logout reg.mosaicstack.dev
+    when:
+      - branch: [main, develop]
+        event: [push, manual]
+    depends_on:
+      - build
+
   docker-build-api:
-    image: plugins/docker
+    image: woodpeckerci/plugin-docker-buildx
     settings:
       registry: reg.mosaicstack.dev
       repo: mosaic/api
       dockerfile: apps/api/Dockerfile
       context: .
+      platforms:
+        - linux/amd64
       tags:
         - "${CI_COMMIT_SHA:0:8}"
         - latest
@@ -102,15 +125,17 @@ steps:
       - branch: [main, develop]
         event: [push, manual]
     depends_on:
-      - build
+      - docker-auth-test
 
   docker-build-web:
-    image: plugins/docker
+    image: woodpeckerci/plugin-docker-buildx
     settings:
       registry: reg.mosaicstack.dev
       repo: mosaic/web
       dockerfile: apps/web/Dockerfile
       context: .
+      platforms:
+        - linux/amd64
       build_args:
         - NEXT_PUBLIC_API_URL=https://api.mosaicstack.dev
       tags:
@@ -124,15 +149,17 @@ steps:
       - branch: [main, develop]
         event: [push, manual]
     depends_on:
-      - build
+      - docker-auth-test
 
   docker-build-postgres:
-    image: plugins/docker
+    image: woodpeckerci/plugin-docker-buildx
     settings:
       registry: reg.mosaicstack.dev
       repo: mosaic/postgres
       dockerfile: docker/postgres/Dockerfile
       context: docker/postgres
+      platforms:
+        - linux/amd64
       tags:
         - "${CI_COMMIT_SHA:0:8}"
         - latest
@@ -144,4 +171,4 @@ steps:
       - branch: [main, develop]
         event: [push, manual]
     depends_on:
-      - build
+      - docker-auth-test