Web assets are built in the node stage, copied into the go stage before
go build so //go:embed picks them up, then the minimal slim runtime
carries only the final binary and ffmpeg.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Builder uses golang:1.23-bookworm (matches runner-base:go-ci).
Runtime is debian:bookworm-slim with ffmpeg (server spec §3) plus a
non-root minstrel user. Release workflow builds from this Dockerfile.
bvandeusen