Permission denied running with docker

### Before submitting an issue, please be sure to

- [x] Read [the contributing instructions](https://github.com/github/pages-gem/blob/master/docs/CONTRIBUTING.md#contributing-to-the-github-pages-gem)
- [x] Update to the latest Gem version (run `bundle update github-pages`)

### This issue affects

- [ ] The site generated by GitHub Pages
- [x] Building sites locally

*The GitHub Pages Gem is intended to help users replicate the GitHub Pages build environment locally. If your issue affects both the hosted version and the version previewed locally, you may be better suited reporting [seeking support in other forums](https://github.com/github/pages-gem/blob/master/docs/SUPPORT.md).*

### What did you do (e.g., steps to reproduce)
```
source contrib/func.sh
cd ../my-site-path/
github-pages
```

### What did you expect to happen?
My site running in a container

### What happened instead?
```
github-pages:3: command not found: realpath
Configuration file: none
Source: /src/site
Destination: /src/site/_site
Incremental build: disabled. Enable with --incremental
Generating...
done in 0.006 seconds.
Auto-regeneration: enabled for '/src/site'
jekyll 3.7.4 | Error: Permission denied @ dir_s_mkdir - /src/site/_site
```
### Additional information

* Link to the live site (if applicable):
* Link to the source repo (if applicable):