Remove env var DOCKER_HOST for Docker.mountDockerSocket.

Checking /var/run/docker.sock is the default anyway.
The env var causes an issue with spring boot maven plugin when running maven inside a container:
spring-projects/spring-boot#22300

Unfortunately, unsetting the var with "withEnv(['DOCKER_HOST='])" does not work, but setting an env var would work.

So we remove the env var here, if anyone should need it (even though it's the default behavior!) it can just be set using
"withEnv(['DOCKER_HOST=unix:///var/run/docker.sock'])".

src/com/cloudogu/ces/cesbuildlib/Docker.groovy

@@ -300,7 +300,7 @@ class Docker implements Serializable {
                 String groupPath = writeGroup()
                 extendedArgs +=
                         // Mount the docker socket
-                        " -v /var/run/docker.sock:/var/run/docker.sock -e DOCKER_HOST=\"unix:///var/run/docker.sock\" " +
+                        " -v /var/run/docker.sock:/var/run/docker.sock " +
                                 // Mount the docker group
                                 "-v ${script.pwd()}/${groupPath}:/etc/group:ro --group-add ${readDockerGroupId()} "
             }

test/com/cloudogu/ces/cesbuildlib/DockerTest.groovy

@@ -274,7 +274,7 @@ class DockerTest {
 
         // extended arg mounts
         assert args[0].contains('-v /home/jenkins/.jenkins/etc/passwd:/etc/passwd:ro ')
-        assert args[0].contains('-v /var/run/docker.sock:/var/run/docker.sock -e DOCKER_HOST=\"unix:///var/run/docker.sock\" -v /home/jenkins/.jenkins/etc/group:/etc/group:ro --group-add 999 ')
+        assert args[0].contains('-v /var/run/docker.sock:/var/run/docker.sock -v /home/jenkins/.jenkins/etc/group:/etc/group:ro --group-add 999 ')
         assert args[0].contains("-v $expectedHome/.jenkins/docker/docker:/usr/bin/docker")
 
         // Docker installed
@@ -314,7 +314,7 @@ class DockerTest {
 
         // extended arg mounts
         assert args[0].contains('-v /home/jenkins/.jenkins/etc/passwd:/etc/passwd:ro ')
-        assert args[0].contains('-v /var/run/docker.sock:/var/run/docker.sock -e DOCKER_HOST=\"unix:///var/run/docker.sock\" -v /home/jenkins/.jenkins/etc/group:/etc/group:ro --group-add 999 ')
+        assert args[0].contains('-v /var/run/docker.sock:/var/run/docker.sock -v /home/jenkins/.jenkins/etc/group:/etc/group:ro --group-add 999 ')
 
         // Written files
         assert 'jenkins:x:1000:1000::/home/jenkins:/bin/sh' == actualWriteFileArgs['.jenkins/etc/passwd']