Set selinux label when mounting docker volumes

When using selinux, docker volumes need to be mounted with :Z in order
to work properly.

Bug: treasure-data#1663

digdag-standards/src/main/java/io/digdag/standards/command/DockerCommandExecutor.java

@@ -113,7 +113,7 @@ private Process startDockerProcess(final CommandContext context,
 
             // mount
             command.add("-v").add(String.format(ENGLISH,
-                        "%s:%s:rw", projectPath, projectPath));  // use projectPath to keep pb.directory() valid
+                        "%s:%s:rw,Z", projectPath, projectPath));  // use projectPath to keep pb.directory() valid
 
             // working directory
             final Path workingDirectory = getAbsoluteWorkingDirectory(context, request); // absolute