Usage with external server: driver default via ADMINER_DEFAULT_DRIVER

[bluepuma77]

According to Docker Hub doc only the default database can be set with ADMINER_DEFAULT_SERVER.

It would be great to also have ADMINER_DEFAULT_DRIVER, ADMINER_DEFAULT_DB and ADMINER_DEFAULT_USER.

Is that possible?

[strowi]

I gave it a try and created a MR adding ADMINER_DEFAULT_DRIVER and ADMINER_DEFAULT_DBNAME. This surely can be extended further..