ci: run only lint and tests on main, build images on tags and test branches

Dockerfile

@@ -1,7 +1,7 @@
 # ==============================================================================
-# Stage 1: Frontend Build (React + TypeScript + Vite + Tailwind CSS)
+# Stage 1: Frontend Build (Runs natively on $BUILDPLATFORM — zero QEMU overhead)
 # ==============================================================================
-FROM node:22-alpine AS frontend-builder
+FROM --platform=$BUILDPLATFORM node:22-alpine AS frontend-builder
 WORKDIR /app/frontend
 COPY frontend/package*.json ./
 RUN npm install --no-audit --no-fund
@@ -9,9 +9,11 @@ COPY frontend/ ./
 RUN npm run build
 
 # ==============================================================================
-# Stage 2: Go Backend Build (Single Static Binary with embed.FS)
+# Stage 2: Go Backend Build (Native Cross-Compile via TARGETOS / TARGETARCH)
 # ==============================================================================
-FROM golang:1.23-alpine AS backend-builder
+FROM --platform=$BUILDPLATFORM golang:1.23-alpine AS backend-builder
+ARG TARGETOS=linux
+ARG TARGETARCH=amd64
 WORKDIR /app
 RUN apk add --no-cache git ca-certificates
 COPY go.mod ./
@@ -21,7 +23,7 @@ COPY migrations/ ./migrations/
 # Copy compiled frontend SPA assets into cmd/server/dist for go:embed
 COPY --from=frontend-builder /app/frontend/dist ./cmd/server/dist
 RUN go mod tidy
-RUN CGO_ENABLED=0 GOOS=linux go build -ldflags="-s -w" -o /app/mindsift ./cmd/server
+RUN CGO_ENABLED=0 GOOS=${TARGETOS} GOARCH=${TARGETARCH} go build -ldflags="-s -w" -o /app/mindsift ./cmd/server
 
 # ==============================================================================
 # Stage 3: Minimal Production Runtime (< 35 MB)