Support auth_dbname through env variable

[darunrs]

PgBouncer has a nifty feature called auth_dbname which allows you to specify the database which it will use for calling auth_query rather than default to trying to call the function on the DB to be connected to. This requires specifying the auth_dbname value for each database under [databases]. I cannot currently do so through the image as the env variable isn't utilized.

Here's an example of auth_dbname in use below:
* = host=postgres port=5432 auth_user=pgbouncer auth_dbname=postgres

It's described on the config page at the bottom of the auth section. It was introduced in this release.

I'd love if the feature could be supported through this image!

I believe implementation would be something like adding ${AUTH_DBNAME:+auth_dbname=${AUTH_DBNAME}} to this line.

[jflambert]

Actually @darunrs my understanding is that there are two possibilities. One is to add auth_dbname to connection strings under [databases], the other is to add is as a global parameter under [pgbouncer].

I'll add AUTH_DBNAME env var support for the global setting. Since we do not yet support multiple databases (see #67) it's as good as it gets for now.

Try latest or v1.23.1-p2 and let me know please.