Add support to configure Initial Image from global configuration

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

@@ -7,6 +7,7 @@
 import hudson.model.Computer;
 import hudson.model.TaskListener;
 import hudson.util.ArgumentListBuilder;
+import jenkins.model.Jenkins;
 import org.apache.commons.io.LineIterator;
 import org.apache.commons.io.output.TeeOutputStream;
 import org.apache.commons.lang.StringUtils;
@@ -255,11 +256,16 @@ private String getDocker0Ip(Launcher launcher, String image) throws IOException,
         // On some distributions, docker doesn't start docker0 bridge until a container do require it
         // So let's run the container once, running /bin/true so it terminates immediately
 
+        DockerBuildWrapper.DescriptorImpl descriptor = (DockerBuildWrapper.DescriptorImpl)
+                Jenkins.getInstance().getDescriptor(DockerBuildWrapper.class);
+
+        final String initImage = descriptor.getInitImage();
+
         ArgumentListBuilder args = dockerCommand()
                 .add("run", "--rm")
                 .add("--entrypoint")
                 .add("/bin/true")
-                .add("alpine:3.6");
+                .add(initImage);
 
         int status = launcher.launch()
                 .envs(getEnvVars())
@@ -290,7 +296,7 @@ private String getDocker0Ip(Launcher launcher, String image) throws IOException,
                 .add("run", "--tty", "--rm")
                 .add("--entrypoint")
                 .add("/sbin/ip")
-                .add("alpine:3.6")
+                .add(initImage)
                 .add("route");
 
         ByteArrayOutputStream out = new ByteArrayOutputStream();

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

@@ -7,6 +7,7 @@
 import hudson.EnvVars;
 import hudson.Extension;
 import hudson.Launcher;
+import hudson.Util;
 import hudson.model.AbstractBuild;
 import hudson.model.AbstractProject;
 import hudson.model.BuildListener;
@@ -18,16 +19,20 @@
 import hudson.remoting.Callable;
 import hudson.tasks.BuildWrapper;
 import hudson.tasks.BuildWrapperDescriptor;
+import hudson.util.FormValidation;
 import hudson.util.ListBoxModel;
 import jenkins.authentication.tokens.api.AuthenticationTokens;
 import jenkins.model.Jenkins;
 import jenkins.security.MasterToSlaveCallable;
+import net.sf.json.JSONObject;
 import org.jenkinsci.plugins.docker.commons.credentials.DockerRegistryToken;
 import org.jenkinsci.plugins.docker.commons.credentials.DockerServerEndpoint;
 import org.kohsuke.stapler.AncestorInPath;
 import org.kohsuke.stapler.DataBoundConstructor;
 import org.kohsuke.stapler.QueryParameter;
+import org.kohsuke.stapler.StaplerRequest;
 
+import javax.servlet.ServletException;
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.util.ArrayList;
@@ -259,6 +264,36 @@ private String whoAmI(Launcher launcher) throws IOException, InterruptedExceptio
     @Extension
     public static class DescriptorImpl extends BuildWrapperDescriptor {
 
+        private String initImage;
+
+        public DescriptorImpl(){
+            super(DockerBuildWrapper.class);
+            load();
+        }
+
+        @Override
+        public boolean configure(StaplerRequest req, JSONObject json) throws FormException {
+            req.bindParameters(this);
+            String paramValue = json.getString("initImage");
+            this.initImage = !paramValue.trim().isEmpty() ? paramValue : "alpine:3.6";
+            save();
+            return super.configure(req,json);
+        }
+
+        public String getInitImage() {
+            return initImage;
+        }
+
+        public void setInitImage(String initImage) {
+            this.initImage = initImage;
+        }
+
+        public FormValidation doCheckInitImage(@QueryParameter String initImage) throws IOException, ServletException {
+            if (Util.fixEmptyAndTrim(initImage) == null)
+                return FormValidation.error("Please set a Init Image name. Default: alpine:3.6");
+            return FormValidation.ok();
+        }
+
         @Override
         public String getDisplayName() {
             return "Build inside a Docker container";

src/main/resources/com/cloudbees/jenkins/plugins/docker_build_env/DockerBuildWrapper/global.jelly

@@ -0,0 +1,32 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+The MIT License
+
+Copyright 2015 CloudBees Inc.
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.
+-->
+<?jelly escape-by-default='true'?>
+<j:jelly xmlns:j="jelly:core" xmlns:f="/lib/form" xmlns:st="jelly:stapler" xmlns:d="/lib/docker/commons" xmlns:c="/lib/credentials">
+      <f:section title="Build In Docker Configuration">
+          <f:entry field="initImage" title="Init image">
+            <f:textbox default="alpine:3.6"/>
+          </f:entry>
+      </f:section>
+</j:jelly>

src/main/resources/com/cloudbees/jenkins/plugins/docker_build_env/DockerBuildWrapper/help-initImage.html

@@ -0,0 +1,4 @@
+<div>
+    This image is used to create two temporary containers to start docker0 bridge and to discover gateway IP from the container.
+    <p> DEFAULT: `alpine:3.6` (since it has a size of 2MB).
+</div>