Set IPC socket group ownership to addon (#134)

Co-authored-by: Mattias Axelsson <[email protected]>

app/dockerdwrapperwithcompose.c

@@ -561,12 +561,13 @@ static const char* build_daemon_args(const struct settings* settings, AXParamete
     if (use_ipc_socket) {
         g_strlcat(msg, " with IPC socket and", msg_len);
         uid_t uid = getuid();
-        uid_t gid = getgid();
-        // The socket should reside in the user directory and have same group as user
+        // The socket should reside in the user directory and have same group as user.
+        // If omitted, dockerd will log a warning about the 'docker' group not being find.
+        // However, rootlesskit maps the user's primary group to the root group, so "--group 0"
+        // means the socket will belong to the user's primary group.
         args_offset += g_snprintf(args + args_offset,
                                   args_len - args_offset,
-                                  " --group %d -H unix:///var/run/user/%d/docker.sock",
-                                  gid,
+                                  " --group 0 -H unix:///var/run/user/%d/docker.sock",
                                   uid);
     } else {
         g_strlcat(msg, " without IPC socket and", msg_len);