I can download large files from my pi at 900mb/s and upload around 800mb/s. The hard drives attached to the pie are formatted ext3. If you use an NTFS drive though, you will get speeds around what you are describing.
Ok, that depends on the docker images; things like pihole are relatively small (300 MB, including all layers); I would be more afraid of running containers over NFS slowed down by rpi ethernet, than downloading the images themselves.
Agreed. I'd have it set up so that there is some local storage via a USB3 thumb drive to hold the container images, which anecdotally pretty easily hits 80MB/s for reads, often hovering in the 100-110MB/s range.