Change image rule to copy config instead of symlink

The image rule in container/image.bzl was symlinking the config file for
strutcture tests. This caused remote execution with
--remote_download_minimal to fail as symlinks aren't yet supported.

container/image.bzl

@@ -506,12 +506,11 @@ def _impl(
     )
     _assemble_image_digest(ctx, name, container_parts, output_tarball, output_digest)
 
-    # Symlink config file for usage in structure tests
-    ln_path = config_file.path.split("/")[-1]
+    # Copy config file for usage in structure tests
     ctx.actions.run_shell(
         outputs = [output_config],
         inputs = [config_file],
-        command = "ln -s %s %s" % (ln_path, output_config.path),
+        command = "cp %s %s" % (config_file.path, output_config.path),
     )
 
     runfiles = ctx.runfiles(