Parity check must read all disks in the parity array.
If any of your docker files (docker.img, appdata) are on the array then your dockers will be impacted. Typically you want your system, domains, and appdata shares to be cache-prefer, and to have all of their files on cache. If any of these files are on the array then docker performance will be impacted due to the slower writes to the parity array, and your dockers will keep parity and array disk(s) spinning. And of course, parity check will be competing for the same disks.
You can easily see which disks any user share is using by clicking Compute... for the share on the User Shares page.