Add a credential helper to avoid plain-text storage warnings

[matt-richardson]

The docker login was emitting a warning:

Download package mcr.microsoft.com/dotnet/sdk v8.0 directly from Docker Registry v1 'Docker Hub' at https://index.docker.io
WARNING! Your credentials are stored unencrypted in 'octo-docker-configs/config.json'. 
Configure a credential helper to remove this warning. See 
https://docs.docker.com/go/credential-store/ 
Login Succeeded 
docker pull mcr.microsoft.com/dotnet/sdk:8.0 
8.0: Pulling from dotnet/sdk 
Digest: sha256:7fd287cec03e027e39be13fb06ca1347f1338c7a6ab4ad305d94d91852693f70 
Status: Image is up to date for mcr.microsoft.com/dotnet/sdk:8.0 
mcr.microsoft.com/dotnet/sdk:8.0 
Staged mcr.microsoft.com/dotnet/sdk version 8.0 (807.973 MB) from Docker Registry v1 'Docker Hub' at https://index.docker.io. 

This was annoying me...

This PR adds a credential helper to handle the credentials nicely.

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⚠️ Does this change require a corresponding Server Change?

• yes, it needs a feature toggle slug added

⚠️ If so - please add a "Requires Server Change" label to this PR!
:done: