Cache misses from differing commit hash leading to unnecessary builds

[mthalman]

There are a few Dockerfiles which are not being recognized as a cache hit to an already published version of the image.

From the build log (internal link):

Image info's Dockerfile commit: https://github.com/dotnet/dotnet-buildtools-prereqs-docker/blob/507c1c3291de8f64d9592f72ae92f8f9f0b45353/src/ubuntu/22.04/coredeps/Dockerfile
Latest Dockerfile commit: https://github.com/dotnet/dotnet-buildtools-prereqs-docker/blob/70a3638e31ff3f7f69a9373b0b4c06773f5eaee6/src/ubuntu/22.04/coredeps/Dockerfile
Dockerfile commits match: False
CACHE MISS

Image info's Dockerfile commit: https://github.com/dotnet/dotnet-buildtools-prereqs-docker/blob/3fc5553956a6e28906d81758fc2852c6a1fd93cc/src/ubuntu/18.04/crossdeps/Dockerfile
Latest Dockerfile commit: https://github.com/dotnet/dotnet-buildtools-prereqs-docker/blob/70a3638e31ff3f7f69a9373b0b4c06773f5eaee6/src/ubuntu/18.04/crossdeps/Dockerfile
Dockerfile commits match: False
CACHE MISS

Image info's Dockerfile commit: https://github.com/dotnet/dotnet-buildtools-prereqs-docker/blob/507c1c3291de8f64d9592f72ae92f8f9f0b45353/src/ubuntu/22.04/coredeps/Dockerfile
Latest Dockerfile commit: https://github.com/dotnet/dotnet-buildtools-prereqs-docker/blob/70a3638e31ff3f7f69a9373b0b4c06773f5eaee6/src/ubuntu/22.04/coredeps/Dockerfile
Dockerfile commits match: False
CACHE MISS

These are being detected as being different commit hashes than what Dockerfile was last used to publish. But that's false because the different commit hashes are actually pointing to the same version of the Dockerfile. For example, with that first one (https://github.com/dotnet/dotnet-buildtools-prereqs-docker/blob/70a3638e31ff3f7f69a9373b0b4c06773f5eaee6/src/ubuntu/22.04/coredeps/Dockerfile), if you go to the URL in your browser, you'll see the webpage refer to that version using the short hash of the equivalent hash:

It's not clear why there are differences in the values, nor why the publishing process doesn't update the image info JSON content with the value that we're detecting the file to have.

[lbussell]

[Triage] My suspicion is that something is up with this command:
https://github.com/dotnet/docker-tools/blob/8fa03e26afbf7676865f2335c0b2f78baf5dbfe9/src/Microsoft.DotNet.ImageBuilder/src/GitHelper.cs#L43

Something must be causing it to retrieve the wrong commit. In the commit it returned, 70a3638, there were no changes to the coredeps Dockerfile.