Compose: Add status service for yelling about misconfiguration

[bisgardo]

It may not always be obvious from the output of Docker Compose if/how a deployment is misconfigured. For example, when the txlog profile is enabled, the env var TXLOG_PGPASSWORD must be provided. If not, the container crashes with an error that requires at least a bit of internal knowledge to make sense of:

error: Invalid value for '--db <config>': invalid connection string: unexpected EOF

Also, it's very easy to miss this message in a flood of logs and not notice that the service stopped.

The status service should do some consistency checks on startup to see if the environment seems consistent. If so, it may exit successfully. Otherwise, it will continuously emit logs to explain what it detected.

An extension could be to stay on and continuously monitor the health of the deployment using whatever Docker/Compose API/SDK enables this. At this point the service might start turning into the dashboard proposed in #23.