Docker add environment variable for mode

[NOTE]: # ( ^^ Provide a general summary of the request in the title above. ^^ )

## Summary

On your docker hub site you mentioned in the Caveats section the following: `Please note that CouchDB no longer autocreates system databases for you, as it is not known at startup time if this is a single-node or clustered CouchDB installation`. With an environment variable this could be done. right?

[NOTE]: # ( Provide a brief overview of what the new feature is all about. )

## Desired Behaviour

Make it possible to define the mode via an environment variable like this:

```shell
docker run -e COUCHDB_MODE=SINGLE|CLUSTERED -e COUCHDB_USER=admin -e COUCHDB_PASSWORD=password couchdb
```

[NOTE]: # ( Tell us how the new feature should work. Be specific. )
[TIP]: # ( Do NOT give us access or passwords to your actual CouchDB! )

## Possible Solution

[NOTE]: # ( Not required. Suggest how to implement the addition or change. )
Adapt the dockerfile to create the required databases for single mode if the environment variable `SINGLE` is present.

## Additional context

This feature matters to me so that newcomers have a easier time setting up couchdb.

[TIP]: # ( Why does this feature matter to you? What unique circumstances do you have? )

Review the Dockerfile and the CouchDB image startup path to see how environment variables and system database creation are handled. Add support for COUCHDB_MODE=SINGLE or CLUSTERED, then verify with docker run that single mode creates the required databases while clustered mode does not.

Written by the indexing model from the issue text.