chore: cross-compile Go binary for docker image to speed up CI

Author: psviderski

.github/workflows/test-build.yaml

@@ -61,7 +61,7 @@ jobs:
             type=semver,pattern={{version}}
             type=raw,value=latest
 
-      - name: Build and push
+      - name: Build and push Docker image (tagged and latest) to GitHub Container Registry
         if: ${{ startsWith(github.ref, 'refs/tags/') }}
         uses: docker/build-push-action@263435318d21b8e681c14492fe198d362a7d2c83 # v6.18.0
         with:

Dockerfile

@@ -1,6 +1,6 @@
 ARG ALPINE_VERSION=3.22.0
 
-FROM golang:1.24-alpine AS builder
+FROM --platform=$BUILDPLATFORM golang:1.24-alpine AS builder
 
 WORKDIR /build
 
@@ -9,7 +9,7 @@ COPY go.mod go.sum ./
 RUN go mod download && go mod verify
 
 COPY . .
-RUN go build -o unregistry ./cmd/unregistry
+RUN GOOS=${TARGETOS} GOARCH=${TARGETARCH} go build -o unregistry ./cmd/unregistry
 
 
 # Unregistry in Docker-in-Docker image for e2e tests.