ci: cache Docker layers between image builds

apk, pnpm install and the migration venv re-ran on every build. Use the
type=gha buildx cache, scoped per matrix image so api and web don't evict
each other.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitea/workflows/build.yml

@@ -104,6 +104,8 @@ jobs:
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
           platforms: linux/amd64
+          cache-from: type=gha,scope=${{ matrix.image }}
+          cache-to: type=gha,mode=max,scope=${{ matrix.image }}
           build-args: |
             APP_VERSION=${{ steps.meta.outputs.version }}
             GIT_SHA=${{ github.sha }}