I think the image size for SQL is about 250 MB, but I’m not sure how much total space it needs for the install process itself. Since you have a PFC200 (not a PFC300 like shown by WagoDamRud), the ~900M free you have in /home/docker isn’t surprising.
You can move the directory for Docker from internal flash to the SD card using the method shown on page 12 of this Docker Quickstart: How To: Getting Started with Docker
But first, I fully agree with WagoDamRud, make sure you are not booting the PFC200 from a SD card firmware image!