Add support for userspace to docker build commands. (#23988)

lib/python/qmk/cli/userspace/path.py

@@ -0,0 +1,8 @@
+from milc import cli
+from qmk.constants import QMK_USERSPACE
+
+
+@cli.subcommand('Detected path to QMK Userspace.', hidden=True)
+def userspace_path(cli):
+    print(QMK_USERSPACE)
+    return