fix(docker): resolve /app/data permission denied on volume mounts

Docker named volumes and host bind-mounts may be owned by root,
causing "open data/model_pricing.sha256: permission denied" when
the container runs as the non-root sub2api user.

Add an entrypoint script that fixes /app/data ownership before
dropping to sub2api via su-exec. Replace USER directive with the
entrypoint approach across all three Dockerfiles and update both
GoReleaser configs to include the script in Docker build contexts.

.goreleaser.simple.yaml

@@ -47,6 +47,8 @@ dockers:
       - "ghcr.io/{{ .Env.GITHUB_REPO_OWNER_LOWER }}/sub2api:latest"
     dockerfile: Dockerfile.goreleaser
     use: buildx
+    extra_files:
+      - deploy/docker-entrypoint.sh
     build_flag_templates:
       - "--platform=linux/amd64"
       - "--label=org.opencontainers.image.version={{ .Version }}"