fix: check that docker is installed on local add

cli/server/add.go

@@ -3,6 +3,7 @@ package server
 import (
 	"context"
 	"errors"
+	"os/exec"
 	"os/user"
 	"strings"
 
@@ -69,6 +70,9 @@ All communication between Abra and the server will use this SSH connection.
 			if err := server.CreateServerDir(domainName); err != nil {
 				logrus.Fatal(err)
 			}
+			if _, err := exec.LookPath("docker"); err != nil {
+				return errors.New("docker command not found on $PATH, is it installed?")
+			}
 			logrus.Info("local server has been added")
 			return nil
 		}