Fix pattern matching for Image Builder path argument

The file pattern matching that is done is based on the string contents of the manifest.json file rather than the physical file system. This requires there to be some non-intuitive workarounds in order to get the desired result for certain cases. For example, when wanting to target the centos/7 images at https://github.com/dotnet/dotnet-buildtools-prereqs-docker/tree/master/src/centos/7, you actually need to use a path filter of `src/centos/7*` rather than `src/centos/7/*`. That's because the pattern matching is use the value of string defined in the manifest.json, `"dockerfile": "src/centos/7"`, to see whether it's a match. If you use the additional slash after the 7, it causes it not to match even though, from a file system sense, it should match.