List of issues with running 2 or more builder pods in a cluster

This issue tracks the current limitations of builder as a clustered system
- It uses a filesystem based lockfile to prevent 2 or more `git push`es at the same time
- In a multi-builder setup, this means that multiple `git push`es can in fact happen at the same time
- To solve, the lock will need to be distributed
- It keeps git repositories on disk and runs pre-receive hooks on them to deploy
- In a multi-builder setup, this means that one `git push` may only have to push a few refs, while another identical `git push` may have to push all refs. Not the end of the world
- To solve, the refs list will need to be moved off of local disk to a shared location (object storage?)