Allow to use unix sockets as URL (jenkinsci#206, JENKINS-23301)

- allow to directly connect to the container without going through
  the mapped port on the host

This is useful if Jenkins itself runs as a container with the host's
Docker socket mapped[1]. To use the docker plugin in this case simply
set the Docker URL to "unix:///var/run/docker.sock".

[1] https://jpetazzo.github.io/2015/09/03/do-not-use-docker-in-docker-for-ci/

.gitignore

@@ -4,3 +4,4 @@ work
 *.iml
 /target
 .*
+*~

docker-plugin/src/main/java/com/nirima/jenkins/plugins/docker/DockerBuildVariableContributor.java

@@ -31,9 +31,11 @@ public void buildVariablesFor(AbstractBuild build, Map<String, String> variables
             variables.put("JENKINS_CLOUD_ID", dockerComputer.getCloudId());
             try {
                 //replace http:// and https:// from docker-java to tcp://
-                final String dockerHost = new URIBuilder(dockerComputer.getCloud().serverUrl)
-                        .setScheme("tcp")
-                        .toString();
+                final URIBuilder uriBuilder = new URIBuilder(dockerComputer.getCloud().serverUrl);
+                if (uriBuilder.getScheme() == "http" || uriBuilder.getScheme() == "https") {
+                    uriBuilder.setScheme("tcp");
+                }
+                final String dockerHost = uriBuilder.toString();
                 variables.put("DOCKER_HOST", dockerHost);
             } catch (URISyntaxException e) {
                 LOG.error("Can't build 'DOCKER_HOST' var: {}", e.getMessage());

docker-plugin/src/main/java/com/nirima/jenkins/plugins/docker/launcher/DockerComputerSSHLauncher.java

@@ -123,6 +123,11 @@ public PortUtils getPortUtils(String cloudId, DockerTemplate dockerTemplate, Ins
         //get address, if docker on localhost, then use local?
         if (host == null || host.equals("0.0.0.0")) {
             host = URI.create(DockerCloud.getCloudByName(cloudId).serverUrl).getHost();
+            if (host == null || host.equals("0.0.0.0")) {
+                // Try to connect to the container directly (without going through the host)
+                host = networkSettings.getIpAddress();
+                port = sshConnector.port;
+            }
         }
 
         return PortUtils.canConnect(host, port);

docker-plugin/src/main/resources/com/nirima/jenkins/plugins/docker/DockerCloud/help-serverUrl.html

@@ -1,3 +1,3 @@
 <div>
-    The URL to use to access your Docker server API (e.g: http://172.16.42.43:4243). Only http:// supported.
+    The URL to use to access your Docker server API (e.g: http://172.16.42.43:4243 or unix:///var/run/docker.sock).
 </div>