Consolidate NuGet package tokens

[spikeheap]

Describe this PR

This PR removes two environment variables from CI which have been used to carry credentials to publish/read packages to our GitHub Packages NuGet registry:

• LBHPACKAGESTOKEN
• NUGET_KEY

LBHPACKAGESTOKEN continues to be used for local development.

What is the problem we're trying to solve

Historically we've published packages from our local machines, which requires a token to authenticate with the GitHub Packages NuGet Registry. Now we use CI to publish packages there is a GitHub-managed token we can use instead..

If you're using a registry that supports granular permissions, and your workflow is using a personal access token to authenticate to the registry, then we highly recommend you update your workflow to use the GITHUB_TOKEN.
~ from https://docs.github.com/en/packages/managing-github-packages-using-github-actions-workflows/publishing-and-installing-a-package-with-github-actions#authenticating-to-package-registries-with-granular-permissions

This change removes both LBHPACKAGESTOKEN and NUGET_KEY tokens from the GitHub Actions workflow, replacing them where needed with the managed GITHUB_TOKEN token that's automatically made available to all jobs.

In order to keep the local development/management experience the same, references to LBHPACKAGESTOKEN have been kept as-is in the Docker and Docker Compose setup.

Docker's documentation suggests not to use build arguments to pass secrets, so this change updates the Dockerfile to use secret mounts, and the recommended way to manage secrets in docker compose.

Consequences

This will allow us to remove the shared secrets in GitHub Actions:

• NUGET_KEY
• LBHPACKAGESTOKEN

At the same time, this doesn't affect the local development workflow.

Checklist

• Code pipeline builds correctly

Follow up actions after merging PR

• Remove this repository from the LBHPACKAGESTOKEN and NUGET_KEY lists here: https://github.com/organizations/LBHackney-IT/settings/secrets/actions.
• Apply the same changes to the
  
  other 9 repositories in that list: