Check to see if Docker buildx is supported when using build features like `--secret` that require it

[danegsta]

If a build attempts to force buildx but it isn't installed, the user experience is a 125 exit code with no other indications what's wrong (see microsoft/aspire#10015). We should do some housekeeping to check for buildx in the Docker implementation if build fails when buildx features are used and report a better error.