Dockerfile: multi-stage build for extension image

[HerbHall]

Description

Rewrite the stub Dockerfile into a proper multi-stage build: Go backend + React frontend + Alpine final image.

Deliverables

• Stage 1: golang:1.22-alpine — cross-compile Go backend with TARGETOS/TARGETARCH
• Stage 2: node:22-alpine — build React frontend with npm run build
• Stage 3: alpine:3.19 — copy binary + built UI + metadata + icon
• Update Makefile if needed for new build flow

Technical Notes

• CGO_ENABLED=0 for static binary
• --platform=$BUILDPLATFORM for cross-compilation
• CMD starts /packetdeck-backend
• UI files go to /ui directory (matching metadata.json ui.dashboard-tab.root)
• Keep extension labels (KG#83)
• .hadolint.yaml for DL3048/DL3045 ignores (KG#78)

Acceptance Criteria

• docker build -t packetdeck:test . succeeds
• docker buildx build --platform=linux/amd64,linux/arm64 . succeeds
• docker extension validate packetdeck:test passes
• docker extension install packetdeck:test installs and opens

Wave

Wave 3 (sequential). Depends on #2, #4.