Use volume rather than mount on Docker invocation

The reason for having a bind mount is unknown. In order to be able to
run this container fom inside another, the directory must be a volume.

Changelog: None

Signed-off-by: Lluis Campos <lluis.campos@northern.tech>

docker-mender-convert

@@ -21,7 +21,7 @@ IMAGE_NAME=${IMAGE_NAME:-mender-convert}
 MENDER_CONVERT_DIR="$(pwd)"
 
 docker run \
-       --mount type=bind,source="$MENDER_CONVERT_DIR,target=/mender-convert" \
+       -v $MENDER_CONVERT_DIR:/mender-convert \
        --privileged=true \
        --cap-add=SYS_MODULE \
        -v /dev:/dev \