Support rebuilding arbitrary images without rebuilding their dependencies

## Steps to reproduce the issue

Try to run a build in the `dotnet-docker` repo with a path argument of `'*sdk*'`. That results in the following error:

```
Error: No such object: dotnetdocker.azurecr.io/build-staging/2289062/dotnet/aspnet:6.0.23-alpine3.18-amd64
```
(From https://dev.azure.com/dnceng/internal/_build/results?buildId=2289062&view=logs&j=fc59f0f2-c1bd-58ae-b870-833d1e8a924c&t=8d53baa0-ff68-5ea5-041a-af0e08303d7f [internal link])

This is because ImageBuilder is trying to build the SDK image when we haven't built its dependency (the ASP.NET image) in the same pipeline run.

## Expected behavior

ImageBuilder should determine when we want to rebuild an arbitrary image in a hierarchy without rebuilding the image it's based on. It should pull the image from MCR without overriding the `REPO` argument (or overriding it as necessary in the nigthly branch, for example).