Reference image permissions prevent running backstop approve

My reference images are all owned by root with `-rw-r--r--` permissions. I use docker for my tests and have backstopjs installed locally to my project. When I try to run `approve` with:

```
backstop approve --config=tests/backstop/config.js --docker
```

I get the following error regarding file permissions:

```
COMMAND | Command "approve" ended with an error after [0.004s]
COMMAND | Error: EACCES: permission denied, unlink '/long/path/to/reference/image.png'
```

`test` and `reference` commands work as expected.

How can I ensure that the image files created by `reference` will have permissions that allow `approve` to work?

Start with tests/backstop/config.js and reproduce the Docker-based reference and approve commands shown in the issue. Trace how the reference command creates image files and how approve removes them, focusing on the reported root ownership and permissions. Done means reference-generated images can be processed by backstop approve without EACCES.

Written by the indexing model from the issue text.