Exposed project directory and project id to the build environment

Exposing the project directory and ID will help with automating Docker integration, for example, mapping a directory in a Docker to the build directory, etc.

lib/build.rb

@@ -143,6 +143,9 @@ module GitlabCi
@process.environment['CI_BUILD_REF_NAME'] = @ref_name
@process.environment['CI_BUILD_ID'] = @id
+      @process.environment['CI_PROJECT_ID'] = @project_id
+      @process.environment['CI_PROJECT_DIR'] = project_dir
+
@process.start
@tmp_file_path = @tmp_file.path