A couple of improvements when running jenkins with docker

Fix automatic jobs creation when starting jenkins service. Due to different user permissions on the mounted volume, git now refuses to clone a repository from the volume if not explicitly marked as safe.

Add a script to configure and start local jenkins with docker, it ensures same docker group id as the host is used in the jenkins docker image or jenkins jobs will end up with errors when executed.

Update instructions in the README.

docker/create_swh_jobs.groovy

@@ -12,22 +12,6 @@ def jobXml = '''<?xml version='1.1' encoding='UTF-8'?>
 <project>
   <description></description>
   <keepDependencies>false</keepDependencies>
-  <scm class="hudson.plugins.git.GitSCM" plugin="git@5.2.1">
-    <configVersion>2</configVersion>
-    <userRemoteConfigs>
-      <hudson.plugins.git.UserRemoteConfig>
-        <url>file:///opt/swh-jenkins-jobs</url>
-      </hudson.plugins.git.UserRemoteConfig>
-    </userRemoteConfigs>
-    <branches>
-      <hudson.plugins.git.BranchSpec>
-        <name>*/master</name>
-      </hudson.plugins.git.BranchSpec>
-    </branches>
-    <doGenerateSubmoduleConfigurations>false</doGenerateSubmoduleConfigurations>
-    <submoduleCfg class="empty-list"/>
-    <extensions/>
-  </scm>
   <canRoam>true</canRoam>
   <disabled>false</disabled>
   <blockBuildWhenDownstreamBuilding>false</blockBuildWhenDownstreamBuilding>
@@ -36,7 +20,12 @@ def jobXml = '''<?xml version='1.1' encoding='UTF-8'?>
   <concurrentBuild>false</concurrentBuild>
   <builders>
     <hudson.tasks.Shell>
-      <command>tox -- update --delete-old --jobs-only</command>
+      <command>
+        git config --global --add safe.directory /opt/swh-jenkins-jobs/.git
+        git clone file:///opt/swh-jenkins-jobs
+        cd swh-jenkins-jobs
+        tox -- update --delete-old --jobs-only
+      </command>
       <configuredLocalRules/>
     </hudson.tasks.Shell>
   </builders>