Reinstate requirements.txt file in docker build

When adding the `dodal` dependency, the docker build was failing because the version stored in the requirements.txt file appeared to be different from the one on `doodal` main branch.
Temporary solution was to remove `-r lockfiles/requirements.txt` from the `PIP_OPTIONS`.

Ideally, it would be good to figure out why this happens (for future dependency needs), fix it and reinstate it.