ci: switch buildx cache from registry to local volume

Replaces type=registry cache (which causes 400 errors from Docker Hub
due to stale buildx builders) with type=local backed by a named Docker
volume on the runner. Adds cache rotation step to prevent unbounded growth.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/docker-build.yml

@@ -20,6 +20,9 @@ on:
 jobs:
   build:
     runs-on: ubuntu-latest
+    container:
+      volumes:
+        - pd-buildx-cache:/opt/buildx-cache
 
     steps:
       - name: Checkout code
@@ -58,8 +61,13 @@ jobs:
           tags: |
             manticorum67/paper-dynasty-discordapp:${{ steps.version.outputs.version }}
             manticorum67/paper-dynasty-discordapp:${{ steps.version.outputs.environment }}
-          cache-from: type=registry,ref=manticorum67/paper-dynasty-discordapp:buildcache
+          cache-from: type=local,src=/opt/buildx-cache/pd-discord
-          cache-to: type=registry,ref=manticorum67/paper-dynasty-discordapp:buildcache,mode=max
+          cache-to: type=local,dest=/opt/buildx-cache/pd-discord-new,mode=max
+
+      - name: Rotate cache
+        run: |
+          rm -rf /opt/buildx-cache/pd-discord
+          mv /opt/buildx-cache/pd-discord-new /opt/buildx-cache/pd-discord
 
       - name: Build Summary
         run: |