make update-admin-pip-requirements does not output files to the working directory

[zenmonkeykstop]

Description

The update-pip-requirements Makefile goal should update all requirements files for the project, but does not update requirements files under /admin/, as the admin Makefile uses a separate Docker image based on Debian Stretch with no shared volume with the host.

Steps to Reproduce

Update a dependency in admin/requirements.in and run make update-pip-requirements

Expected Behavior

The file admin/requirements.txt should include the updated dependency.

Actual Behavior

The file admin/requirements.txt is untouched.