image-info.json's `commitUrl` is incorrect when manifest is not in repo root

Example: https://github.com/dotnet/versions/blob/ff091bf000d8cbdd8aa8822b2122f536b6f9a562/build-info/docker/image-info.dotnet-docker-tools-main-imagebuilder.json#L29

https://github.com/dotnet/dotnet-docker/blob/30a3bfe0a3bbe1baae8ed261081f97147fc6c275/Dockerfile.linux is not a valid location. It should be https://github.com/dotnet/docker-tools/blob/30a3bfe0a3bbe1baae8ed261081f97147fc6c275/src/Dockerfile.linux instead.

(Unrelated to this issue, it looks like this repo has the git repo URL set incorrectly as well)

The calculation for `commitUrl` computes the path relative to the manifest file, but the git URL calculation assumes that it is relative to the root of the repo.

**Suggested fix**: Update Dockerfile path calculation to be relative to the root of the repo.