Error running backstop test with docker container

Here's my configuration file

```
{
"id": "test",
"viewports": [
{
"label": "small",
"width": 320,
"height": 480
},
{
"label": "medium",
"width": 568,
"height": 1024
},
{
"label": "large",
"width": 1024,
"height": 768
},
{
"label": "xlarge",
"width": 1291,
"height": 768
}
],
"scenarios": [
{
"label": "Family Circle Story",
"url": "http://www.familycircle.com/family-fun/10-books-to-read-now/",
"hideSelectors": [
"#ad",
"#mobileHoverBannerWrapper",
"#header-banner-container",
"#sidebar-second"
],
"removeSelectors": [
"#block-outbrain-integration-outbrain",
"#monetate_lightbox",
"#block-medianet-medianet"
],
"selectorExpansion": true,
"selectors": [],
"readyEvent": null,
"delay": 1000,
"misMatchThreshold" : 10,
"requireSameDimensions" : true
}
],
"paths": {
"bitmaps_reference": "backstop_data/bitmaps_reference",
"bitmaps_test": "backstop_data/bitmaps_test",
"engine_scripts": "backstop_data/engine_scripts",
"html_report": "backstop_data/html_report",
"ci_report": "backstop_data/ci_report"
},
"engineFlags": [],
"engine": "chrome",
"report": ["browser"],
"debug": true,
"debugWindow": false
}
```

and after trying to run this using the container via `docker run --rm -v $(pwd):/src docksal/backstopjs test`, I get the following error
```
CasperJS: [info] [phantom] Step anonymous 8/28: done in 5491ms.
CasperJS: [info] [phantom] Step anonymous 9/28 http://www.familycircle.com/family-fun/10-books-to-read-now/ (HTTP 200)
CasperJS: [info] [phantom] Step anonymous 9/28: done in 5508ms.
CasperJS: [info] [phantom] Step anonymous 10/28 http://www.familycircle.com/family-fun/10-books-to-read-now/ (HTTP 200)
CasperJS: [debug] [phantom] Navigation requested: url=about:blank, type=Other, willNavigate=true, isMainFrame=true
CasperJS: [debug] [phantom] url changed to "about:blank"

Testing script failed with code: 1

An unexpected error occured. You may want to try setting the debug option to `true` in your config file.
COMMAND | Command `test` ended with an error
COMMAND | Error: An unexpected error occured. You may want to try setting the debug option to `true` in your config file.
at ChildProcess. (/usr/lib/node_modules/backstopjs/core/util/createBitmaps.js:70:18)
at emitTwo (events.js:106:13)
at ChildProcess.emit (events.js:191:7)
at maybeClose (internal/child_process.js:877:16)
at Process.ChildProcess._handle.onexit (internal/child_process.js:226:5)
```
When run the tests on my local machine (mac OS) it runs fine. Also I don't understand why inside the container it uses casperjs instead of chrome.

If you have any pointers, it will be appreciated.

Thanks