Implement more robust check if buildx is supported by docker (#774)
Implement more robust check if buildx is supported by docker (#774)

There was a bug in the existing checker (checked if a variable was
non-zero length instead of zero length).  I fixed that but also
made the check a little more robust.
scalyr/scalyr-agent-2,scalyr/scalyr-agent-2,scalyr/scalyr-agent-2,scalyr/scalyr-agent-2