Normally I'm all for people using tried and tested primitives for things, however I think that in this case unix sockets are probably not the right choice.

Firstly you are creating a hard dependency for having the two services sharing a same box, with a shared file system (that's difficult to coordinate and secure.) But also should you add a new service that also want to connect via unix socket, stuff could get tricky to orchestrate.

But this also limits your ability to move stuff about, should you need it.

Inside a container, I think its probably a perfectly legitimate way to do IPC. Between containers, I suspect you are asking for trouble.