Move PublishAsDockerFile to use container resources.

[davidfowl]

Description

This allows full customization of the docker building process and removes a dependency on the limited docker.v0 resource type.

• Updated DockerfileBuildAnnotation to allow nullable build arguments.
• Modified WithBuildArg to accept nullable values.
• Revised tests in ManifestGenerationTests.cs and PublishAsDockerfileTests.cs to align with new behavior.
• Adjusted SchemaTests.cs to use temporary context paths for Dockerfile testing.

Fixes #4067

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
    • Is this introducing a breaking change?
      • Yes
        • Link to aspire-docs issue (please use this breaking-change template):
      • No
        • Link to aspire-docs issue (please use this doc-idea template):
  • No