Compose: Add override for logging transactions into Dockerized DB

[bisgardo]

Transaction logging is enabled with a profile in the main spec. It currently only supports logging transactions into an external PostgreSQL instance. An override txlog-local (which activates the txlog profile) would enable them to be ingested into a DB container running as part of the deployment instead.

Related to #59.

[bisgardo]

When done, update mention in https://github.com/bisgardo/concordium-docker/blob/67d4d2cd61a70ed3eeb4e668b070587f5b620530/postgresql.md#setting-up-postgresql.