.circleci/config.yml: fix image tag in s2i-builder job
.circleci/config.yml: fix image tag in s2i-builder job

The problem is that in PRs, `CIRCLE_BRANCH` includes a '/'. For example,
for PR 567, it equals `pull/567`. That's an issue because in Docker,
tags cannot contain '/'.

This commits fixes the issue by using only the PR number.

3scale/docker-gateway,3scale/docker-gateway,3scale/apicast,3scale/apicast,3scale/apicast,3scale/apicast