The previous regex could mistake user provided pdf files, like
`fake_output.pdf`, as the final output file.
The frontend expects to find a `output.pdf` file on success.
- move setting into clsi.docker namespace
- rename the variable for images to allowedImages / ALLOWED_IMAGES
- add an additional check for the image name into the DockerRunner
Co-Authored-By: Brian Gough <brian.gough@overleaf.com>