Changes

PR #2091 by jimscard and ksylvan: Update Dockerfile for best practices and critical CVE fixes

• Pins Alpine 3.21 and Go 1.25.9 explicitly for reproducible, auditable builds.
• Installs the Go toolchain directly in the builder stage, removing the dependency on an unavailable upstream golang tag.
• Upgrades setuptools to remediate the critical vulnerability CVE-2025-47273.
• Refreshes the yt-dlp installation path to align with current packaging conventions.
• Configures the final image to run as a non-root user, reducing the container's attack surface.