Allow relative path in mounted dirs

[Matthieu]

Currently, the syntax bind:rw:/my/own/datadir:/var/lib/postgresql/data only allows absolute paths for the host path.
It should be easy to accept relative paths (transforming them into absolute paths before the Docker call)

[Dan Crosta]

You can use tox substitutions to achieve the same result with greater explicitness, eg https://github.com/tox-dev/tox-docker/blob/master/tox.ini#L61

[Matthieu]

You're right, thanks for the tip!