scripts: add proper 386 and amd64 target triples and builds

We need these to match the Makefile detection of the right gcc for
runc-dmz, as well as making sure that everything builds properly for our
cross-i386 tests. While we're at it, add x86 to the list of build
targets for release builds (presumably nobody will use it, but since we
do test builds of this anyway it probably won't hurt).

In addition, clean up the handling of the native architecture build by
treating it the same as any other build (ensuring that building runc
from a different platform will work the same way regardless of the
native architecture). In practice, the build works the same way as
before.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

Dockerfile

@@ -9,19 +9,15 @@ ARG CRIU_REPO=https://download.opensuse.org/repositories/devel:/tools:/criu/Debi
 RUN KEYFILE=/usr/share/keyrings/criu-repo-keyring.gpg; \
     wget -nv $CRIU_REPO/Release.key -O- | gpg --dearmor > "$KEYFILE" \
     && echo "deb [signed-by=$KEYFILE] $CRIU_REPO/ /" > /etc/apt/sources.list.d/criu.list \
+    && dpkg --add-architecture i386 \
     && apt-get update \
     && apt-get install -y --no-install-recommends \
         build-essential \
         criu \
-        gcc-aarch64-linux-gnu libc-dev-arm64-cross \
-        gcc-arm-linux-gnueabi libc-dev-armel-cross \
-        gcc-arm-linux-gnueabihf libc-dev-armhf-cross \
-        gcc-powerpc64le-linux-gnu libc-dev-ppc64el-cross \
-        gcc-s390x-linux-gnu libc-dev-s390x-cross \
-        gcc-riscv64-linux-gnu libc-dev-riscv64-cross \
+        gcc \
+        gcc-multilib \
         curl \
         gawk \
-        gcc \
         gperf \
         iptables \
         jq \
@@ -32,6 +28,14 @@ RUN KEYFILE=/usr/share/keyrings/criu-repo-keyring.gpg; \
         sudo \
         uidmap \
         iproute2 \
+    && apt-get install -y --no-install-recommends \
+        libc-dev:i386 libgcc-s1:i386 \
+        gcc-aarch64-linux-gnu libc-dev-arm64-cross \
+        gcc-arm-linux-gnueabi libc-dev-armel-cross \
+        gcc-arm-linux-gnueabihf libc-dev-armhf-cross \
+        gcc-powerpc64le-linux-gnu libc-dev-ppc64el-cross \
+        gcc-s390x-linux-gnu libc-dev-s390x-cross \
+        gcc-riscv64-linux-gnu libc-dev-riscv64-cross \
     && apt-get clean \
     && rm -rf /var/cache/apt /var/lib/apt/lists/* /etc/apt/sources.list.d/*.list
 
@@ -54,7 +58,7 @@ RUN cd /tmp \
 ARG LIBSECCOMP_VERSION
 COPY script/seccomp.sh script/lib.sh /tmp/script/
 RUN mkdir -p /opt/libseccomp \
-    && /tmp/script/seccomp.sh "$LIBSECCOMP_VERSION" /opt/libseccomp arm64 armel armhf ppc64le riscv64 s390x
+    && /tmp/script/seccomp.sh "$LIBSECCOMP_VERSION" /opt/libseccomp 386 amd64 arm64 armel armhf ppc64le riscv64 s390x
 ENV LIBSECCOMP_VERSION=$LIBSECCOMP_VERSION
 ENV LD_LIBRARY_PATH=/opt/libseccomp/lib
 ENV PKG_CONFIG_PATH=/opt/libseccomp/lib/pkgconfig