我尝试按照这个tutorial来创建一个简单的Docker环境，作为Jenkins管道构建的一部分。
我正在尝试构建几个Docker映像，作为在我进行Maven构建之前的测试。目前，我的Jenkinsfile有以下Groovy：

# !groovy
node {
  stage 'Building docker env'
  def dbImage = docker.build('oracle', 'docker/oracle')
  def wlpImage = docker.build('liberty', 'docker/liberty')
  stage 'Running maven build'
  git url: 'https://mysite/myproject.git', branch: 'docker'
  def mvnHome = tool 'maven 3.3.9'
  sh "${mvnHome}/bin/mvn -B clean install"
}

我尝试让Docker构建在目录“docker/oracle”中查找并调用该目录中的Dockerfile，然后构建名为oracle和liberty的Docker映像。目前，它给我以下错误：

Running on master in /root/.jenkins/workspace/pipeline_test
[Pipeline] {
[Pipeline] stage (Building docker env)
Using the ‘stage’ step without a block argument is deprecated
Entering stage Building docker env
Proceeding
[Pipeline] }
[Pipeline] // node
[Pipeline] End of Pipeline
groovy.lang.MissingPropertyException: No such property: docker for class: groovy.lang.Binding
    at groovy.lang.Binding.getVariable(Binding.java:63)
    at org.jenkinsci.plugins.scriptsecurity.sandbox.groovy.SandboxInterceptor.onGetProperty(SandboxInterceptor.java:224)
    at org.kohsuke.groovy.sandbox.impl.Checker$4.call(Checker.java:241)
    at org.kohsuke.groovy.sandbox.impl.Checker.checkedGetProperty(Checker.java:238)
    at org.kohsuke.groovy.sandbox.impl.Checker.checkedGetProperty(Checker.java:221)
    at com.cloudbees.groovy.cps.sandbox.SandboxInvoker.getProperty(SandboxInvoker.java:28)
    at com.cloudbees.groovy.cps.impl.PropertyAccessBlock.rawGet(PropertyAccessBlock.java:20)
...

你知道我使用的docker build命令有什么问题吗？（或者是我忘记在Jenkins中安装的东西？）