Diagnostic checks for network connectivity issues

Hello, I am trying to diagnose a network connectivity issue with my company's internal-only satis repository when connected via VPN. Everything works fine when plugged into the network directly (I don't think this is a relevant detail but this is within the `docker build` process), but when attempting to run `composer install` while connected via VPN I see the following:

```
Step 21 : RUN composer install --prefer-source --no-autoloader
---> Running in 12abca38a5e9
Loading composer repositories with package information

[Composer\Downloader\TransportException]
The "https://satis.mycompany.com/packages.json" file could not be downloaded: failed to open stream: Connection timed out

install [--prefer-source] [--prefer-dist] [--dry-run] [--dev] [--no-dev] [--no-plugins] [--no-custom-installers] [--no-autoloader] [--no-scripts] [--no-progress] [-v|vv|vvv|--verbose] [-o|--optimize-autoloader] [--ignore-platform-reqs] [packages1] ... [packagesN]
```

My composer.json begins with:

```
{
"repositories": [
{
"type": "composer",
"url": "https://satis.mycompany.com"
},
```

This is what `composer diag` looks like:

```
Step 21 : RUN composer diag
---> Running in ab12c34a4c87
Checking composer.json: FAIL
the property name is required
the property description is required
No license specified, it is recommended to do so. For closed-source software you may use "proprietary" as license.
require.some/repo : unbound version constraints (dev-master) should be avoided
Checking platform settings: OK
Checking git settings: OK
Checking http connectivity to packagist: OK
Checking https connectivity to packagist: OK
Checking github.com oauth access: OK
Checking disk free space: OK
Checking composer version: OK
```

Oddly enough, I am able to `curl https://satis.mycompany.com/packages.json` directly.

My main question: is there any way to run the type of http connectivity check `composer diag` does for packagist, for a local satis repo?

Thanks in advance for any suggestions you guys can provide.