Ability to set default credentials

For example https://hub.docker.com/r/michalhosna/adminer allows you to set credentials like this:

```
ADMINER_DB: adminer
ADMINER_DRIVER: pgsql
ADMINER_PASSWORD: adminer
ADMINER_SERVER: postgres
ADMINER_USERNAME: adminer
ADMINER_AUTOLOGIN: 1
ADMINER_NAME: This will be in the title!
```

Unfortunately that image is not updated.

It would be very useful to be able to do the same in this image.