Cross-compile multi-platform images without QEMU

Dockerfile

@@ -1,18 +1,31 @@
-FROM golang:alpine AS builder
+# syntax=docker/dockerfile:1
+FROM --platform=$BUILDPLATFORM golang:alpine AS builder
 
 WORKDIR /src
+
+RUN apk add --update --no-cache --no-progress make git
+COPY go.mod go.sum ./
+RUN --mount=type=cache,target=/go/pkg/mod go mod download
 COPY . /src
 
-RUN apk add --update --no-cache --no-progress make git \
-    && make server
+ARG TARGETOS
+ARG TARGETARCH
+RUN --mount=type=cache,target=/go/pkg/mod \
+    --mount=type=cache,target=/root/.cache/go-build \
+    GOOS=$TARGETOS GOARCH=$TARGETARCH make server
+
+# Certificates and timezone data are architecture-independent. Prepare them
+# natively so the target image needs no RUN instructions or QEMU emulation.
+FROM --platform=$BUILDPLATFORM alpine:latest AS runtime-data
+RUN apk add --update --no-cache --no-progress ca-certificates tzdata
 
 FROM alpine:latest
 LABEL org.opencontainers.image.licenses=Apache-2.0
 LABEL org.opencontainers.image.source="https://gitea.bytevibe.dev/gary/metatube-server"
 
 COPY --from=builder /src/build/metatube-server .
-
-RUN apk add --update --no-cache --no-progress ca-certificates tzdata
+COPY --from=runtime-data /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/ca-certificates.crt
+COPY --from=runtime-data /usr/share/zoneinfo /usr/share/zoneinfo
 
 ENV GIN_MODE=release
 ENV PORT=8080