print out build container logs when verbose is enabled

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

@@ -367,4 +367,17 @@ private List<String> dockerCommandArgs() {
         }
         return args;
     }
+
+    public void logs(String container) throws IOException, InterruptedException {
+        ArgumentListBuilder args = dockerCommand()
+            .add("logs", "-t", container);
+
+        listener.getLogger().println("Retrieving container logs");
+        OutputStream out = listener.getLogger();
+        OutputStream err = listener.getLogger();
+        int status = launcher.launch()
+                .cmds(args)
+                .stdout(out).stderr(err).quiet(true).join();
+        if (status != 0) listener.error("Unable to retrieve logs from container");
+    }
 }

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

@@ -192,7 +192,16 @@ public Environment setUp(AbstractBuild build, final Launcher launcher, BuildList
         return new Environment() {
             @Override
             public boolean tearDown(AbstractBuild build, BuildListener listener) throws IOException, InterruptedException {
-                return build.getAction(BuiltInContainer.class).tearDown();
+                BuiltInContainer builtIn = build.getAction(BuiltInContainer.class);
+                if ( verbose ) {
+                    try {
+                        builtIn.getDocker().logs(builtIn.container);
+                    }
+                    catch ( Exception ex ) {
+                        LOGGER.log(Level.WARNING, "Error getting container logs", ex);
+                    }
+                }
+                return builtIn.tearDown();
             }
         };
     }