allow use of config file in container

[HPPinata]

This logic checks whether bridgeData.yaml has been mapped to it's standard location.
If it finds a file there, it uses it instead of relying on env variables.

Apart from ENV variables this is the other common way of importing dynamic configuration into a container.
It comes with the advantage of not having to recreate the container to change the config and not having to convert anything, at the expense of having two state files (compose and bridgeData) and having to do a bindmount.

This should work, but hasn't been tested extensively, thus the draft.
Depending on whether this, or the ROCm Dockerfile are merged first I'll update the correct pull request to match the others status.

I should also update the documentation to reflect that this is an option.