Native cross compilation

Dockerfile

@@ -1,38 +1,47 @@
 ARG MSMTP_VERSION=1.8.16
+ARG ALPINE_S6_VERSION=3.14-2.2.0.3
+ARG XX_VERSION=1.0.0-rc.2
 
-FROM --platform=${BUILDPLATFORM:-linux/amd64} crazymax/alpine-s6:3.14-2.2.0.3 AS download
-RUN apk --update --no-cache add curl tar unzip xz
+FROM --platform=$BUILDPLATFORM tonistiigi/xx:${XX_VERSION} AS xx
+FROM --platform=$BUILDPLATFORM crazymax/alpine-s6:${ALPINE_S6_VERSION} AS builder
+RUN apk --update --no-cache add \
+    clang \
+    curl \
+    pkgconf \
+    tar \
+    unzip \
+    xz \
+  && rm -rf /tmp/*
 
 ARG MSMTP_VERSION
-WORKDIR /dist/msmtp
+WORKDIR /src
 RUN curl -sSL "https://marlam.de/msmtp/releases/msmtp-$MSMTP_VERSION.tar.xz" | tar xJv --strip 1
 
-FROM crazymax/alpine-s6:3.14-2.2.0.3 AS builder
-RUN apk --update --no-cache add \
-    autoconf \
-    automake \
-    binutils \
-    build-base \
+ARG TARGETPLATFORM
+ENV XX_CC_PREFER_LINKER=ld
+COPY --from=xx / /
+RUN xx-apk --no-cache add \
+    file \
+    g++ \
     gettext-dev \
     gnutls-dev \
+    libc-dev \
     libidn2-dev \
     libgsasl-dev \
     libsecret-dev \
-    openssl-dev \
-  && rm -rf /tmp/*
-
-COPY --from=download /dist/msmtp /tmp/msmtp
-WORKDIR /tmp/msmtp
-RUN ./configure \
+    make
+RUN set -x \
+  && ./configure \
+    --host=$(xx-clang --print-target-triple) \
     --prefix=/usr \
     --sysconfdir=/etc \
     --mandir=/usr/share/man \
     --localstatedir=/var \
   && make -j$(nproc) \
-  && make install \
-  && msmtp --version
+  && make install
+RUN msmtp --version
 
-FROM crazymax/alpine-s6:3.14-2.2.0.3
+FROM crazymax/alpine-s6:${ALPINE_S6_VERSION}
 
 ENV S6_BEHAVIOUR_IF_STAGE2_FAILS="2" \
   TZ="UTC" \
@@ -51,7 +60,6 @@ RUN apk --update --no-cache add \
     libgsasl \
     libsecret \
     mailx \
-    openssl \
     shadow \
     tzdata \
   && ln -sf /usr/bin/msmtp /usr/sbin/sendmail \