Allow docker build secrets to be files

[eerhardt]

Description

This allows people to pass .npmrc files as docker secrets without baking auth credentials into this image.

Checklist

• Is this feature complete?
  • Yes. Ready to ship.
  • No. Follow-up changes expected.
• Are you including unit tests for the changes and scenario tests if relevant?
  • Yes
  • No
• Did you add public API?
  • Yes
    • If yes, did you have an API Review for it?
      • Yes
      • No
    • Did you add <remarks /> and <code /> elements on your triple slash comments?
      • Yes
      • No
  • No
• Does the change make any security assumptions or guarantees?
  • Yes
    • If yes, have you done a threat model and had a security review?
      • Yes
      • No
  • No
• Does the change require an update in our Aspire docs?
  • Yes
    • Link to aspire.dev issue:
      • New issue
  • No

[github-actions]

🚀 Dogfood this PR with:

⚠️ WARNING: Do not do this without first carefully reviewing the code of this PR to satisfy yourself it is safe.

curl -fsSL https://raw.githubusercontent.com/dotnet/aspire/main/eng/scripts/get-aspire-cli-pr.sh | bash -s -- 14559

Or

• Run remotely in PowerShell:

iex "& { $(irm https://raw.githubusercontent.com/dotnet/aspire/main/eng/scripts/get-aspire-cli-pr.ps1) } 14559"

[mitchdenny]

Do you think we should have `WithBuildSecretFile(callback) where we can give it a file, but defer evaluation.

[karolz-ms]

We should be explicit here and use type=file or type=env, as appropriate https://docs.docker.com/reference/cli/docker/buildx/build/#secret

[karolz-ms]

Compare with https://docs.podman.io/en/latest/markdown/podman-build.1.html (supports the same type designator)