Fix lock & cache interactions
Fix lock & cache interactions

Summary:
Rework how we start the base container. We hold the lock for less time
and now clear out the cache after.

Test Plan: Will deploy this locally to a few machines.

Reviewers: armooo

Reviewed By: armooo

Subscribers: mkedia, vishal

Differential Revision: https://tails.corp.dropbox.com/D121626

dropbox/changes-client,dropbox/changes-client