The command is:
docker run -v "$PWD":/var/task "lambci/lambda:build-python3.6" /bin/sh -c "pip install -r /var/task/requirements.txt -t python/lib/python3.6/site-packages/; exit"
And I am running it from the same folder as the requirements.txt file.
I get the following error:
ERROR: Could not open requirements file: [Errno 2] No such file or directory: '/var/task/requirements.txt'
Source: Docker Questions