ci: Use standardized Docker image to build protos

[thanethomson]

Follows from #173.

We need to update the proto-gen Makefile target to build our protos using a standardized Docker image (one we can build and push to our Docker Hub org) so that our whole team and CI all use precisely the same tooling to build the protos.

Similar to how the SDK does it here.

Given that such an image will require mounting a volume from the host system to (1) read the proto files and (2) output generated Go code, I recommend allowing the UID to be set by way of build args, like this image handles it, to cater for non-Mac systems that don't handle mapping of user IDs well. Otherwise the produced files have different user/group IDs in the filesystem on the host system that require root privileges to adjust before they're usable.

[melekes]

@greg-szabo if you're looking for a low-hanging fruit, this might be the one