build: multi-stage Dockerfile with node->go->slim for embedded SPA

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>

Dockerfile

@@ -1,10 +1,19 @@
 # syntax=docker/dockerfile:1.6
 
+FROM node:22-bookworm-slim AS web
+WORKDIR /web
+COPY web/package.json web/package-lock.json ./
+RUN npm ci
+COPY web/ ./
+RUN npm run build
+
 FROM golang:1.23-bookworm AS builder
 WORKDIR /src
 COPY go.mod go.sum ./
 RUN go mod download
 COPY . .
+# Overwrite the committed placeholder with the freshly-built SPA assets.
+COPY --from=web /web/build ./web/build
 ENV CGO_ENABLED=0
 RUN go build -trimpath -ldflags="-s -w" -o /out/minstrel ./cmd/minstrel