Add details for running tests from nightly build images. Closes #24993.

The current documentation guidelines throw an error after running the
commands in the tensorflow nightly build container. Following this
Magenta issue:

https://github.com/tensorflow/magenta/issues/1232

running the test requires changing directory. Rather than changing the
WORKDIR in the image, which would probably affect many uses and users,
this change adds the direction of changing directory after mentioning
the nightly build image.