update docker container to 123

Currently the docker container is still version 122. Also the path isn't optimal as it's not in the PATH by default, so containers based on the diffoscope container can't use it in a convenient way.

Will update the version shortly - I've been on spotty/limited internet so I delayed the upload of this.

Regarding the PATH issue, I'm not a Docker expert but it seems like something like:

ENV PATH="/srv/diffoscope/bin:${PATH}"

would do the trick here? (I don't really want to install diffoscope to /usr/bin)

assigned to @lamby

closed via commit 37dc9345

I've fixed the PATH issue in 37dc9345 and am pushing the new version to the Docker registry as we speak. :)