Merge pull request #938 from chendave/doc

doc: make sure the intended message is redirected to file successfully

frontend/dockerfile/docs/experimental.md

@@ -119,8 +119,9 @@ This mount type allows the build container to access SSH keys via SSH agents, wi
 FROM alpine
 RUN apk add --no-cache openssh-client
 RUN mkdir -p -m 0700 ~/.ssh && ssh-keyscan gitlab.com >> ~/.ssh/known_hosts
-RUN --mount=type=ssh ssh git@gitlab.com | tee /hello
+RUN --mount=type=ssh ssh -q -T git@gitlab.com 2>&1 | tee /hello
 # "Welcome to GitLab, @GITLAB_USERNAME_ASSOCIATED_WITH_SSHKEY" should be printed here
+# with the type of build progress is defined as `plain`.
 ```
 
 ```console