[keyserver] Get rid of container_name for Node in docker-compose.yml

Summary:
From docs:

Because Docker container names must be unique, you cannot scale a service beyond 1 container if you have specified a custom name.

@jimpo also highlighted this in his review of D3713.

Test Plan: N/A

Reviewers: palys-swm, jimpo

Reviewed By: palys-swm

Subscribers: Adrian, atul, karol-bisztyga, yayabosh, jimpo

Differential Revision: https://phabricator.ashoat.com/D3847