Bug in r.27 release tpu/tools/docker/Dockerfile

[Daniel Vega-Myhre]

TensorFlow installation is using v2.2.0: https://github.com/tensorflow/tpu/blob/r2.7/tools/docker/Dockerfile#L27

However, it should be 2.7 since tensorlfow/models and tensorflow/tpu are both using 2.7: https://github.com/tensorflow/tpu/blob/r2.7/tools/docker/Dockerfile#L33-L36

[Daniel Vega-Myhre]

cc @aman2930

[Aman Gupta]

Fixed it.