Enable debug by WITH_DEBUG in gerrit comment

Change-Id: Ia820830f3f32c8129b904f25dd5ef97442c7ca22

.ci/validate_environment.sh

@@ -50,6 +50,11 @@ if [ "${CONTAINER_BUILD_TYPE}" = "testing" ] \
 
     echo "${GERRIT_EVENT_COMMENT_TEXT}" | base64 -d > "${tmpfile_comment}"
 
+    if grep -q "\bWITH_DEBUG\b" "${tmpfile_comment}"; then
+        export YASHCHIKI_DEBUG=enabled
+        set_debug_output_from_env
+    fi
+
     if ! grep -q "\bNO_FAILED_CACHE\b" "${tmpfile_comment}"; then
         if grep -q "\bUSE_CACHE_NAME=" "${tmpfile_comment}"; then
             # use specified cache

README.md

@@ -47,3 +47,8 @@ This changeset adds the possibility to specify:
 These take priority over commit-specified `Depends-On:` and are mutually
 exclusive with jenkins-specified build parameters since each build gets
 either triggered manually in jenkins or via gerrit.
+
+
+### `WITH_DEBUG`
+
+Specifying `WITH_DEBUG` in the triggering comment will enable debug output.