Make tmpfs / ramdisk available within container

We've got some extremely slow performance with rspec tests and mariadb in a docker context, and it would be useful to try making use of a tmpfs directory to speed things up.

I know this is something that can't be done within a docker container without launching it with special admin privileges, which we can't control from within the bitbucket-pipelines.yml file, but maybe something can be made available on your end, with a special option to make use of it in that yaml file.