mysql-client has no install candidate

This seems like a small error, but I got this message:

```
Package mysql-client is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'mysql-client' has no installation candidate
```

I know it's in the docker file, but I figured I'd let you know :)