[ERROR] MYSQL_HOSTNAME env variable is not set, but required

Thank you for the effort to create this image.
```
$ docker run --rm -itp 127.0.0.1:80:80 cytopia/dvwa
[ERROR] MYSQL_HOSTNAME env variable is not set, but required
```
I'm having a problem to run it. Do I need to setup a seperate mysql for this?

Start by reading the repository's Docker Compose setup and the image's documented environment requirements, focusing on MYSQL_HOSTNAME. Reproduce the shown docker run command and compare it with the supported startup path; done means the required MySQL setup and invocation are clearly documented or the reported startup failure is resolved.

Written by the indexing model from the issue text.