Composer can't access local Satis server

Question

I am hosting a Satis Private Repository currently on my local machine while I do some debugging and testing. I have added a repository too which currently just contains a project on GitLab which included a composer package.

Currently it is being pulled from a docker image, and is hosted at localhost:8002.

My repository is setup in my Laravel project as follows:

"repositories": [
    { "packagist": false },
    {
        "type": "composer",
        "url": "http://localhost:8002/packages.json"
    }],

And if I navigate to that URL in my browser I get the following output:

{
    "packages": [],
    "includes": {
        "include/all$949e06d91b8be4b54109a0b06c31805a5735a1ba.json": {
            "sha1": "949e06d91b8be4b54109a0b06c31805a5735a1ba"
        }
    }
}

However, if I try to composer update the Laravel project, I get the following errors depending on if I execute it from within the container.

 The "http://localhost:8002/packages.json" file could not be downloaded (HTTP/1.1 504 Gateway Timeout) 

[Composer\Downloader\TransportException]
  The "http://localhost:8002/packages.json" file could not be downloaded: failed to open stream: Address not available

If anyone would be able to point me in the right direction at all I would really appreciate it, as I am scratching my head here.