将Jenkins凭证（文件）传递给Docker Build Arg

我想将jenkins凭证（机密文件）的内容作为jenkins文件中的build arg传递给docker

正如这里所描述的，我们必须在build arg中对该文件执行cat。但在詹金斯如何做到这一点

--build-arg ssh_key="$(cat ~/.ssh/id_rsa)"

Dockerfile

...
ARG ssh_key
RUN echo "$ssh_key" > /root/.ssh/id_rsa
...

...
environment {
ssh_key = credentials(ssh_key)
}
...

def build = docker.build(registry,"--build-arg ssh_key= .)

Jenkinsfile

...
ARG ssh_key
RUN echo "$ssh_key" > /root/.ssh/id_rsa
...

...
environment {
ssh_key = credentials(ssh_key)
}
...

def build = docker.build(registry,"--build-arg ssh_key= .)

有什么想法吗