install s6 overlay for the right platform

docker/scripts/install-s6.sh

@@ -0,0 +1,29 @@
+#!/usr/bin/env bash
+set -eux pipefail
+
+BUILD_PACKAGES="wget ca-certificates"
+
+apt-get update
+apt-get -y install --no-install-recommends $BUILD_PACKAGES
+
+case `uname -m` in
+    arm*)
+        PLATFORM=armhf
+        ;;
+    aarch64*)
+        PLATFORM=aarch64
+        ;;
+    x86_64*)
+        PLATFORM=amd64
+        ;;
+esac
+
+pushd /tmp
+wget https://github.com/just-containers/s6-overlay/releases/download/v1.21.8.0/s6-overlay-${PLATFORM}.tar.gz
+tar xzf s6-overlay-${PLATFORM}.tar.gz -C /
+rm s6-overlay-${PLATFORM}.tar.gz
+popd
+
+apt-get -y purge --autoremove $BUILD_PACKAGES
+apt-get clean
+rm -rf /var/lib/apt/lists/*