containers: Install requirements.txt in containers

Add requirements.txt to capture all the Python packages that the cli
tool depends on, then install all of those packages when building
containers. This is especially important for the ci container, to allow
fastpath to run.

In order to make this work, we need to expand the build context to cover
the entire repo when building the containers, so that the Dockerfile can
COPY the requirements.txt. Adjust some paths to make this conformant.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>