Use the real branch name, not HEAD

Author: piki

.buildkite/hooks/pre-command

@@ -8,5 +8,5 @@ echo "Semantic version = \"${CONTAINER_SEMVER}\""
 export CONTAINER_SHORT_SHA=$(git log -1 --pretty='%h')
 echo "Short SHA = \"${CONTAINER_SHORT_SHA}\""
 
-export CONTAINER_BRANCH_NAME=$(git rev-parse --abbrev-ref HEAD | sed -e 's/[\/:]/_/g')
+export CONTAINER_BRANCH_NAME=$(echo "$BUILDKITE_BRANCH" | sed -e 's/[\/:]/_/g')
 echo "Safe branch name = \"${CONTAINER_BRANCH_NAME}\""