[terraform] Avoid including Terraform *.tfstate.backup in keyserver docker images

Summary: Files like terraform.tfstate.backup expose secrets in the keyserver image and should be

Test Plan:
Tested by building the docker image, opening the build with docker run -it docker_image bash and
searching for my password cd .. && (grep -R password_string . | grep -v node_modules) as we did in https://phab.comm.dev/D12823

Reviewers: varun, bartek, ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12849