feat: default commands & values from env vars

in gitlab-ci, you can define `DEFAULT_VALUE_` prefixed variables:

DEFAUT_VALUE_application_command: "my command"

which will get converted to

```yaml
application:
  command: my command
```

each value can additionaly be a json string and will then be parsed as such (if possible)

notice that any other values (from files, etc.) will take precedence over these variables
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