CI: Add a debian-stable-i386 CI image

This is identical to the debian-stable image, except that it uses
packages from the i386 dpkg architecture (i686-linux-gnu) instead of
amd64 (x86_64-linux-gnu). x86_64 Docker hosts with x86_64 kernels can
run i386 Docker images, so we can use our existing CI workers.

Instead of duplicating the content of the Dockerfile, add a layer of
architecture-switching so we can build essentially the same image
from a different base.

Signed-off-by: Simon McVittie <smcv@debian.org>

.gitlab-ci/debian-stable.Dockerfile

@@ -1,4 +1,5 @@
-FROM debian:bookworm
+ARG ARCHITECTURE_PREFIX=
+FROM ${ARCHITECTURE_PREFIX}debian:bookworm
 
 RUN apt-get update -qq && apt-get install --no-install-recommends -qq -y \
     bindfs \

.gitlab-ci/run-docker.sh

@@ -85,6 +85,8 @@ if [ $list == 1 ]; then
 
                 echo -e "  \\e[1;39m$basename\\e[0m"
         done
+        # Special cases that don't have their own Dockerfile
+        echo -e "  \\e[1;39mdebian-stable-i386\\e[0m"
         exit 0
 fi
 
@@ -94,8 +96,25 @@ if [ -z "${base}" ]; then
         exit 1
 fi
 
-if [ ! -f "$base.Dockerfile" ]; then
-        echo -e "\\e[1;31mERROR\\e[0m: Dockerfile for '$base' not found"
+case "$base" in
+        (debian-stable-i386)
+            dockerfile=debian-stable
+            arch=386
+            if [ "$DOCKER_CMD" = "podman" ]; then
+                arch_prefix=docker.io/i386/
+            else
+                arch_prefix=i386/
+            fi
+            ;;
+        (*)
+            dockerfile="$base"
+            arch=""
+            arch_prefix=""
+            ;;
+esac
+
+if [ ! -f "$dockerfile.Dockerfile" ]; then
+        echo -e "\\e[1;31mERROR\\e[0m: Dockerfile for '$dockerfile' not found"
         exit 1
 fi
 
@@ -113,8 +132,10 @@ if [ $build == 1 ]; then
                 --build-arg HOST_USER_ID="$UID" \
                 --build-arg COVERITY_SCAN_PROJECT_NAME="${COVERITY_SCAN_PROJECT_NAME}" \
                 --build-arg COVERITY_SCAN_TOKEN="${COVERITY_SCAN_TOKEN}" \
+                ${arch_prefix+--build-arg ARCHITECTURE_PREFIX="${arch_prefix}"} \
+                ${arch+--arch="$arch"} \
                 --tag "${TAG}" \
-                --file "${base}.Dockerfile" .
+                --file "${dockerfile}.Dockerfile" .
         exit $?
 fi
 
@@ -132,6 +153,7 @@ fi
 if [ $run == 1 ]; then
         echo -e "\\e[1;32mRUNNING\\e[0m: ${base} as ${TAG}"
         $DOCKER_CMD run \
+                ${arch+--arch="$arch"} \
                 --rm \
                 --volume "$(pwd)/..:/home/user/app" \
                 --workdir "/home/user/app" \