3.x docker image requires usernames longer than a single character

## Expected Behavior

I expect to be able to set a username via the `COUCHDB_USER` environment variable with a length of 1 character.

## Current Behavior

Previous versions of the docker image did not have any validation on service start. The current versions fails if the specified `COUCHDB_USER` is a single character.

You'll see the following message on container start with a single-character username:

```
*************************************************************
ERROR: CouchDB 3.0+ will no longer run in "Admin Party"
mode. You *MUST* specify an admin user and
password, either via your own .ini file mapped
into the container at /opt/couchdb/etc/local.ini
or inside /opt/couchdb/etc/local.d, or with
"-e COUCHDB_USER=admin -e COUCHDB_PASSWORD=password"
to set it via "docker run".
*************************************************************
```

I believe the cause is the `\w+` in the regex check [here](https://github.com/apache/couchdb-docker/blob/main/3.1.1/docker-entrypoint.sh#L85)

## Possible Solution

I think changing the `\w+` to `\w*` will allow the previous behavior.

## Steps to Reproduce (for bugs)

```shell
docker run -e COUCHDB_USER=l -e COUCHDB_PASSWORD=password -d couchdb
```

Here is an example of downstream usage of the image where it just constantly restarts the container (though the error message isn't surfaced as I don't surface that in the tests):

https://github.com/dokku/dokku-couchdb/runs/3578023307?check_suite_focus=true

## Context

This breaks my tests, which I try to sync between all datastore plugins. Fixing it just means increasing the username length, but its annoying since now I can't easily copy the files over as tests change.

That said, its not a super-critical bug, just an annoying one.

## Your Environment

* Version used: 3.1.1
* Browser Name and version: Safari Version 14.1.2 (16611.3.10.1.3)
* Operating System and version (desktop or mobile): macOS Big Sur 11.5
* Link to your project: https://github.com/dokku/dokku-couchdb