Mount files db in RAM

[splakhonin]

Based on the following documentation it's cool to add the possibility to map database files into RAM.
https://docs.gitlab.com/runner/executors/docker.html#mount-a-directory-in-ram

`[runners.docker]

For the main container

[runners.docker.tmpfs]
"/var/lib/mysql" = "rw,noexec"

For services

[runners.docker.services_tmpfs]
"/var/lib/mysql" = "rw,noexec"`

As docker/sdk generates a docker-compose.yml, here we have a volume or tmpfs mount definition, https://docs.docker.com/compose/compose-file/compose-file-v3/#long-syntax-3

The idea is to add a configuration to switch on/off mounting map database files into RAM.