new: [compose-core] move partial docker-compose to a file

bin/compose-core

@@ -1287,8 +1287,8 @@ get_docker_compose () {
     merge_yaml_str "$(yaml_key_val_str "services" "$docker_compose_services")" \
                    "$base_v2" > "$cache_file" || return 1
 
-    export _current_docker_compose="$(cat "$cache_file")"
-    echo "$_current_docker_compose"
+    export _CURRENT_DOCKER_COMPOSE="$cache_file"
+    cat "$_CURRENT_DOCKER_COMPOSE"
     debug "  ..compilation of base 'docker-compose.yml' done $GRAY(in $((SECONDS - start_compilation))s)$NORMAL" || true
     # debug " ** ${WHITE}docker-compose.yml${NORMAL}:"
     # debug "$_current_docker_compose"
@@ -1392,12 +1392,15 @@ export -f get_service_charm
 ## full docker-compose.yml to be already built.
 get_service_def () {
     local service="$1" def
-    if [ -z "$_current_docker_compose" ]; then
-        print_syntax_error "$FUNCNAME is meant to be called after"\
-                           "\$_current_docker_compose has been calculated."
+    if [ -z "$_CURRENT_DOCKER_COMPOSE" ]; then
+        err "${FUNCNAME[0]} is meant to be called after"\
+            "\$_CURRENT_DOCKER_COMPOSE has been calculated."
+        echo "  Called by:" >&2
+        printf "  - %s\n" "${FUNCNAME[@]:1}" >&2
+        return 1
     fi
 
-    def=$(echo "$_current_docker_compose" | shyaml get-value "services.${service//./\\.}" 2>/dev/null)
+    def=$(cat "$_CURRENT_DOCKER_COMPOSE" | shyaml get-value "services.${service//./\\.}" 2>/dev/null)
     if [ -z "$def" ]; then
         err "No definition for service $DARKYELLOW$service$NORMAL in compiled 'docker-compose.yml'."
         return 1
@@ -3847,7 +3850,14 @@ launch_docker_compose() {
     mkdir -p "$docker_compose_tmpdir/$project"
     docker_compose_dir="$docker_compose_tmpdir/$project"
 
-    get_docker_compose $SERVICE_PACK > "$docker_compose_dir/docker-compose.yml" || return 1
+    if [ -z "$_CURRENT_DOCKER_COMPOSE" ]; then
+        err "${FUNCNAME[0]} is meant to be called after"\
+            "\$_CURRENT_DOCKER_COMPOSE has been calculated."
+        echo "  Called by:" >&2
+        printf "  - %s\n" "${FUNCNAME[@]:1}" >&2
+        return 1
+    fi
+    cat "$_CURRENT_DOCKER_COMPOSE" > "$docker_compose_dir/docker-compose.yml" || return 1
     if [ -e "$state_tmpdir/to-merge-in-docker-compose.yml" ]; then
         # debug "Merging some config data in docker-compose.yml:"
         # debug "$(cat $state_tmpdir/to-merge-in-docker-compose.yml)"