Enabling federation_sender in the worker-enabled docker image causes Complement tests to fail

[anoadragon453]

When adding federation_sender to the list of enabled workers (via the SYNAPSE_WORKERS env var) to the Synapse worker Docker image, Complement tests that rely on receiving federation transactions fail. This is predictably due to the in-built Complement server never receiving any federation transactions from Synapse.

@MatMaul and myself have looked into this, and have so far concluded that:

• send_federation is true for the federation_sender work and false for the master process, as intended.
• The federation_sender is starting up correctly.
• Adding federation_sender_instances to the federation_sender config does not help (nor should it really, as we're only using one federation sender...)

My hunch is that somehow the master process is not communicating transactions that need to be sent out to the federation sender. Additionally, while enabling other workers poses no problems, it's worth noting that Complement still only has a very small number of tests. As such the federation sender may not be the only worker having issues; it may just be the first to show them.

[anoadragon453]

This has been fixed in f6571ee. Turns out we weren't starting redis properly 🤦