Updated documentation.

ci/install-docker.sh

@@ -1,9 +1,13 @@
 #!/usr/bin/env bash
 
+# See: https://docs.travis-ci.com/user/docker/#using-docker-compose
+# See: https://docs.travis-ci.com/user/docker/#installing-a-newer-docker-version
+# See: https://docs.docker.com/engine/install/ubuntu/#next-steps
 sudo apt-get clean
 curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
 sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"
 sudo apt-get update
 sudo apt-get -y -o Dpkg::Options::="--force-confnew" install docker-ce docker-ce-cli containerd.io docker-compose-plugin
+# Print docker and docker compose tool versions.
 docker --version
-docker compose version
+docker compose version