Fix the path to the init.sql file in the mysql-setup container

Unlike the upstream image, we do not store files on the root directory
because of the way that blubber works. For that reason, we need to
modify the location of the init.sql file that is referred to be sed when
initialising the database.

Bug: T329514
Change-Id: I1f57b29e70877452989cc0699d9900678b27b3bc