pass information on Jenkins user to container launch script

Some container launch scripts may need to know the username and user/group IDs
of the Jenkins user which the actual build steps will run as.  In particular
the container launch script may want to provide a background service, such as
a graphical environment, for use during the build.

This change passes in this information as part of the container's process
environment.

src/main/java/com/cloudbees/jenkins/plugins/docker_build_env/DockerBuildWrapper.java

@@ -73,6 +73,16 @@ public class DockerBuildWrapper extends BuildWrapper {
 
     private String cpu;
 
+    /**
+     * Variable name to pass user:group IDs to the container launch script in its environment.
+     */
+    public static final String JENKINS_USERID_VARIABLE = "JENKINS_USER_IDS";
+
+    /**
+     * Variable name to pass username string to the container launch script in its environment.
+     */
+    public static final String JENKINS_USERNAME_VARIABLE = "JENKINS_USER_NAME";
+
     @DataBoundConstructor
     public DockerBuildWrapper(DockerImageSelector selector, String dockerInstallation, DockerServerEndpoint dockerHost, String dockerRegistryCredentials, boolean verbose, boolean privileged,
                               List<Volume> volumes, String group, String command,
@@ -182,7 +192,7 @@ public Environment setUp(AbstractBuild build, final Launcher launcher, BuildList
                 }
             }
 
-            runInContainer.container = startBuildContainer(runInContainer, build, listener);
+            runInContainer.container = startBuildContainer(runInContainer, build, listener, launcher);
             listener.getLogger().println("Docker container " + runInContainer.container + " started to host the build");
         }
 
@@ -199,10 +209,14 @@ public boolean tearDown(AbstractBuild build, BuildListener listener) throws IOEx
 
 
 
-    private String startBuildContainer(BuiltInContainer runInContainer, AbstractBuild build, BuildListener listener) throws IOException {
+    private String startBuildContainer(BuiltInContainer runInContainer, AbstractBuild build, BuildListener listener, final Launcher launcher) throws IOException {
         try {
             EnvVars environment = buildContainerEnvironment(build, listener);
 
+            // provide Jenkins User/Group IDs and User Name to the containter environment
+            environment.putIfNotNull(JENKINS_USERID_VARIABLE, whoAmI(launcher));
+            environment.putIfNotNull(JENKINS_USERNAME_VARIABLE, whoAmI_Name(launcher));
+
             String workdir = build.getWorkspace().getRemote();
 
             Map<String, String> links = new HashMap<String, String>();
@@ -249,6 +263,14 @@ private String whoAmI(Launcher launcher) throws IOException, InterruptedExceptio
 
     }
 
+    private String whoAmI_Name(Launcher launcher) throws IOException, InterruptedException {
+        ByteArrayOutputStream bos = new ByteArrayOutputStream();
+        launcher.launch().cmds("id", "-un").stdout(bos).quiet(true).join();
+        String userName = bos.toString().trim();
+
+        return userName;
+    }
+
     @Extension
     public static class DescriptorImpl extends BuildWrapperDescriptor {
 

src/main/java/com/cloudbees/jenkins/plugins/docker_build_env/DockerDecoratedLauncher.java

@@ -59,6 +59,8 @@ private EnvVars buildContainerEnvironment() throws IOException, InterruptedExcep
             this.env = runInContainer.getDocker().getEnv(runInContainer.container, launcher);
         }
         EnvVars environment = new EnvVars(env);
+        environment.remove(DockerBuildWrapper.JENKINS_USERID_VARIABLE);
+        environment.remove(DockerBuildWrapper.JENKINS_USERNAME_VARIABLE);
 
         // Let BuildWrapper customize environment, including PATH
         for (Environment e : build.getEnvironments()) {