Linux xargs calls the command with no arguments if it gets no inputs, which
`docker rmi` complains about. It provides -r / --no-run-if-empty to prevent
this but unfortunately this isn't supported on OSX.
Ignore errors from `docker rmi` so that `make clean` will keep going and clean
up later stuff.
Signed-off-by: Ian Campbell <ian.campbell@docker.com>
