Move DDEV global config into /workspace where it will be persistent

Problem/Motivation

DDEV's global config is not currently persistent, so if a user makes changes to the global configuration and then later starts the project, any global configuration originally done by DDEV or by the user will be lost.

Proposed resolution

Set XDG_CONFIG_HOME:

```
ENV XDG_CONFIG_HOME=/workspace/.config
```

Remaining tasks

User interface changes

API changes

Data model changes