new: dev: [compose-core] introduce ``is_docker_compose_action_multi_service`` variable for convenience

bin/compose-core

@@ -4667,6 +4667,7 @@ no_init=
 action=
 stage="main"  ## switches from 'main', to 'action', 'remainder'
 is_docker_compose_action=
+is_docker_compose_action_multi_service=
 rebuild_relations_to_service=()
 color=
 declare -A without_relations
@@ -4806,6 +4807,12 @@ while read-0 arg; do
                         fi
                         pos_args=($(echo "$DC_USAGE" | sed -r 's/\[-[^]]+\] ?//g;s/\[options\] ?//g'))
                         pos_args=("${pos_args[@]:1}")
+                        if [[ "${pos_args[0]}" == "[SERVICE...]" ]]; then
+                            is_docker_compose_action_multi_service=1
+                        elif [[ "${pos_args[0]}" == "SERVICE" ]]; then
+                            is_docker_compose_action_multi_service=0
+                        fi
+
                         # echo "USAGE: $DC_USAGE"
                         # echo "pos_args: ${pos_args[@]}"
                         # echo "MULTI: $DC_MATCH_MULTI"
@@ -5006,8 +5013,10 @@ if [ -n "$is_docker_compose_action" ] && [ "${#services_args[@]}" -gt 0 ]; then
             seen[$mservice]=1
             action_posargs+=("$mservice")
         done
-    else
+    elif [ "$is_docker_compose_action_multi_service" == "1" ]; then
         action_posargs+=("${services[@]}")
+    elif [ "$is_docker_compose_action_multi_service" == "0" ]; then
+        action_posargs+=("${services[0]}")  ## only the first service is the legit one
     fi
     ## Get rid of subordinates
     action_posargs=($(get_master_services "${action_posargs[@]}")) || exit 1