`mlserver 0.4.1` used regardless of the local version

[jklaise]

The local version of mlserver should be preferred over the default image (which is 0.4.1), however, this does not seem to be the case. I have confirmed that either using mlserver 0.4.0 or 1.0.1 locally results in a Docker image running 0.4.1 instead. This should be reproducible by running the custom-model example as it specifies mlserver==0.4.0 which is different from 0.4.1 which is ultimately run in the Docker image.

This also has some implications with the behaviour of the Docker environment. E.g. if the local version of mlserver is 1.0.1, which doesn't have a dependency on orjson, the created Docker image of mlserver 0.4.1 doesn't work as orjson is missing, but it is required by mlserver 0.4.1.