[services] Base image - Rename contents to docker

Summary:
https://linear.app/comm/issue/ENG-1121/reconsider-base-imagecontents-directory

Renaming the` contents` folder to the docker folder for the base image.

Test Plan:

cd services
yarn build-base-image

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: ashoat, palys-swm, Adrian, atul, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4000