Automagically use i386/debian:buster (RPi-Distro#415)

* Autmagically use 1386/debian:buster when running on 64-bit host to prevent error RPi-Distro#271

Dockerfile

@@ -1,4 +1,5 @@
-FROM debian:buster
+ARG BASE_IMAGE=debian:buster
+FROM ${BASE_IMAGE}
 
 ENV DEBIAN_FRONTEND noninteractive
 

build-docker.sh

@@ -73,7 +73,17 @@ fi
 # Modify original build-options to allow config file to be mounted in the docker container
 BUILD_OPTS="$(echo "${BUILD_OPTS:-}" | sed -E 's@\-c\s?([^ ]+)@-c /config@')"
 
-${DOCKER} build -t pi-gen "${DIR}"
+# Check the arch of the machine we're running on. If it's 64-bit, use a 32-bit base image instead
+case "$(uname -m)" in
+  x86_64|aarch64)
+    BASE_IMAGE=i386/debian:buster
+    ;;
+  *)
+    BASE_IMAGE=debian:buster
+    ;;
+esac
+${DOCKER} build --build-arg BASE_IMAGE=${BASE_IMAGE} -t pi-gen "${DIR}"
+
 if [ "${CONTAINER_EXISTS}" != "" ]; then
 	trap 'echo "got CTRL+C... please wait 5s" && ${DOCKER} stop -t 5 ${CONTAINER_NAME}_cont' SIGINT SIGTERM
 	time ${DOCKER} run --rm --privileged \