Drop BuildKit cache mounts so legacy builder can build the image

The host runner's Docker CLI forces the buildx path when DOCKER_BUILDKIT=1
is set, but buildx isn't installed on the runner, so the build errored
before starting. Revert the env var and remove the two RUN --mount=type=cache
directives so the legacy builder can parse the Dockerfile. Normal layer
caching still applies; only the npm download cache reuse is lost.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/docker-build.yml

@@ -21,8 +21,6 @@ jobs:
           password: ${{ secrets.REGISTRY_TOKEN }}
 
       - name: Build and Push
-        env:
-          DOCKER_BUILDKIT: "1"
         run: |
           # gitea.repository is already "owner/repo" (e.g. jason/breedr).
           IMAGE="registry.alwisp.com/${{ gitea.repository }}"