I've been having the same issue when i use docker for windows on windows 10. My work around for this was to run the docker daemon in experimental mode.You can do so by following these steps:
- Open Docker in windows
- click on Settings
- click on daemon
- go to advanced
- put the value
"experimental": true
- then restart docker
With this you should be able to pull the PHP image.
To get more acquainted with docker try out docker training course.