fix: [compose-core] prevent re-building images already present

Here, subordinate charms requested images using their names and not
their master's charm name. This would trigger builds of images already
present.

bin/compose-core

@@ -1430,7 +1430,7 @@ service_base_docker_image() {
         ## According to https://stackoverflow.com/questions/32230577 , if there's a build,
         ## then the builded image will get name ${project}_${service}
         project=$(get_default_project_name) || return 1
-        image_name="${project}_${service}"
+        image_name="${project}_${master_service}"
         if ! docker_has_image "$image_name"; then
             service_build=$(echo "$service_def" | shyaml get-value build 2>/dev/null)
             if [ "$?" != 0 ]; then
@@ -1439,12 +1439,12 @@ service_base_docker_image() {
                 return 1
             fi
 
-            docker build "$service_build" -t "${project}_${service}" >&2 || {
-                err "Failed to build image for ${DARKYELLOW}$service${NORMAL}."
+            docker build "$service_build" -t "${image_name}" >&2 || {
+                err "Failed to build image '${image_name}' for ${DARKYELLOW}$service${NORMAL}."
                 return 1
             }
         fi
-        printf "%s" "${project}_${service}"
+        printf "%s" "${image_name}"
     else
         printf "%s" "${service_image}"
     fi | tee "$cache_file"