new: added ``docker_image_export_dir``, ``service_base_image_export_dir`` helper function

This implements a missing feature in docker to allow to export some
part of a docker image.

bin/compose-core

@@ -495,6 +495,30 @@ image_exposed_ports_0() {
 }
 export -f image_exposed_ports_0
 
+
+## feature not yet included in docker: https://github.com/moby/moby/issues/16079
+docker_image_export_dir() {
+    local image="$1" src="$2" dst="$3" container_id
+    (
+        container_id=$(docker create "$image") || exit 1
+        trap_add EXIT,ERR "docker rm \"$container_id\" >/dev/null"
+        docker cp "$container_id":"$src" "$dst"
+    )
+}
+export -f docker_image_export_dir
+
+
+service_base_image_export_dir() {
+    local service="$1" src="$2" dst="$3" base_image
+    shift
+    base_image=$(service_base_docker_image "$service") || return 1
+    if ! docker_has_image "$base_image"; then
+        docker pull "$base_image"
+    fi
+    docker_image_export_dir "$base_image" "$src" "$dst"
+}
+export -f service_base_image_export_dir
+
 ##
 ## Generic
 ##