Fix pip in py3_image (#1244)

Author: lshamis

python3/image.bzl

@@ -118,4 +118,9 @@ def py3_image(name, base = None, deps = [], layers = [], **kwargs):
         args = kwargs.get("args"),
         data = kwargs.get("data"),
         testonly = kwargs.get("testonly"),
+        # The targets of the symlinks in the symlink layers are relative to the
+        # workspace directory under the app directory. Thus, create an empty
+        # workspace directory to ensure the symlinks are valid. See
+        # https://github.com/bazelbuild/rules_docker/issues/161 for details.
+        create_empty_workspace_dir = True,
     )