[Backup] Remove docker context differences

Summary:
Make the docker context look like the repo structure for backup,
then update all assumptions.

Re-ordered rust installation to avoid changes to services directories causing rust to re-install

https://linear.app/comm/issue/ENG-1809

Test Plan:

# In repo
nix develop
cd services/backup
rm -rf build && mkdir build && cd build && cmake .. && make -j

# In docker
docker build . -f services/backup/Dockerfile

backup CI gate should also suffice for docker workflow

Reviewers: atul, varun, abosh, karol, max

Reviewed By: max

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D5156