new: [compose-core] make detection of cached image work with correct syntax

bin/compose-core

@@ -1812,7 +1812,7 @@ service_ensure_image_ready() {
     local service_quoted=${service//./\\.}
 
     if specified_image=$(echo "$service_def" | shyaml get-value image 2>/dev/null); then
-        if [ "$specified_image" == "$charm_image_name"* ]; then
+        if [[ "$specified_image" == "$charm_image_name"* ]]; then
             ## Assume we already did the change
             printf "%s" "$specified_image" | tee "$cache_file"
             return 0