Test specifying dns name for forwarded port

[alexr00]

Refs: #3450

• dev container @roblourens
• dev container @rebornix

Complexity: 5

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If you already have a setup where you want to specify a dns name then feel free to use that! Otherwise, here's out get one:

1. Clone https://github.com/microsoft/vscode-dev-containers
2. Checkout branch alexr00/testPortDns
3. In VS Code, run the command Remote-Containers: Open Folder in Container... and choose this folder from your cloned repo: ./containers/javascript-node-mongo
4. To test that the port is forwarded correctly:
   1. Clone https://github.com/alexr00/test-mongo-connection-project locally
   2. npm install then npm run start and verify that you get the successful connection message.

Things to verify:

1. Verify that you can specify a port with a dns in forwardPorts in devcontainer.json.
2. Verify that you can forward a port from the forwarded ports view with a dns specified (using the above example, enter "db:27017" when forwarding a port).