install: Skip "docker already installed" warning in REPO_ONLY mode

When REPO_ONLY=1, the script only sets up the package repository and
exits. There's no reason to warn about Docker being already installed or
sleep for 20 seconds in that case.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

Author: vvoland

install.sh

@@ -418,7 +418,7 @@ check_forked() {
 do_install() {
 	echo "# Executing docker install script, commit: $SCRIPT_COMMIT_SHA"
 
-	if command_exists docker; then
+	if [ "$REPO_ONLY" != "1" ] && command_exists docker; then
 		cat >&2 <<-'EOF'
 			Warning: the "docker" command appears to already exist on this system.