Before this change:

```
unset DOCKER_BUILDKIT

make build
docker build --build-arg=CROSS=false -t "docker-dev:require-buildkit" -f "Dockerfile" .
Sending build context to Docker daemon 50.01MB
Error response from daemon: Dockerfile parse error line 17: Unknown flag: mount
make: *** [build] Error 1
```

After this change:

```
unset DOCKER_BUILDKIT

make build
docker build --build-arg=CROSS=false -t "docker-dev:require-buildkit" -f "Dockerfile" .
[+] Building 5.2s (71/71) FINISHED
=> [internal] load .dockerignore 0.1s
...
...
=> => exporting layers 0.9s
=> => writing image sha256:1ea4128a0e7f3bdee47de1675252609d9d6071e32da24a2aafee9fba96b2404b 0.0s
=> => naming to docker.io/library/docker-dev:require-buildkit 0.0s
...

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>