build: fix the docs build (googleapis#13444)

The docs build is failing with error `ERROR: Could not open requirements
file: [Errno 2] No such file or directory:
'/workspace/.kokoro/requirements.txt'`. The root of the problem is that
this file was deleted in
googleapis#13433.

I also needed to add the dependency `gcp-docuploader` to resolve the
`/usr/local/bin/python3.10: No module named docuploader` error during
the upload step

This PR resolves the failure by adding the necessary requirements file
under `/.kokoro/docker/docs`

Author: parthea

.kokoro/docker/docs/Dockerfile

@@ -81,4 +81,8 @@ RUN wget -O /tmp/get-pip.py 'https://bootstrap.pypa.io/get-pip.py' \
 # Test pip
 RUN python3.10 -m pip
 
+# Install build requirements
+COPY requirements.txt /requirements.txt
+RUN python3.10 -m pip install --require-hashes -r requirements.txt
+
 CMD ["python3.10"]

.kokoro/docker/docs/requirements.in

@@ -0,0 +1,2 @@
+nox
+gcp-docuploader