Grab it from dockerhub:
docker pull danielkinsman/godot_server_docker
Export a godot project to linux:
docker run -v /home/user/some/directory/:/mnt/working danielkinsman/godot_server_docker godot_server -v -path /mnt/working/godot_project -export "Linux X11" /mnt/working/game
Android (debug):
docker run -v /home/user/some/directory/:/mnt/working danielkinsman/godot_server_docker godot_server -v -path /mnt/working/godot_project -export_debug "Android" /mnt/working/game.apk