Release for Raspberry Pi 4 (Raspbian Buster)

Correct, buster is only in nightly channel. You can use

curl -fsSL get.docker.com | CHANNEL=nightly sh

to install Docker on Raspbian Buster 2019-07-10-raspbian*.img until next stable release of Docker CE.
If you want to install a stable release now, you can use

curl -fsSL get.docker.com | sed 's/buster/stretch/' | sh

which installs 18.09.0 at the moment.

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