fix(docker): non-root user, supply chain attestations, build cleanup

BREAKING: Container now runs as UID 1000 (uptop) instead of root.
Existing volumes with root-owned files need migration:

  docker run --rm -v <volume>:/data alpine chown -R 1000:1000 /data

- Add uptop user (UID/GID 1000) with entrypoint writability check
- Enable SBOM and provenance attestations for Docker Scout compliance
- Prune dangling images and build cache after release builds

docker-entrypoint.sh

@@ -0,0 +1,12 @@
+#!/bin/sh
+set -e
+
+if [ ! -w /data ]; then
+    echo "ERROR: /data is not writable by uptop user (UID $(id -u))." >&2
+    echo "" >&2
+    echo "If upgrading from a previous version that ran as root:" >&2
+    echo "  docker run --rm -v <your_volume>:/data alpine chown -R 1000:1000 /data" >&2
+    exit 1
+fi
+
+exec "$@"