chore: warn if docker secret cleanup fails

[BrunoQuaresma]

Closes #159

[johnstcn]

I think this might also be a good place to add a check in the integration tests that the docker config.json was removed, something like this might work:

		statOutput := execContainer(t, ctr, "stat "+filepath.Join(envbuilder.MagicDir, "config.json"))
		require.Contains(t, statOutput, "No such file or directory")