At the time of creation the FROM image wasn't being tagged, now it is, so we should use the fixed tag (sstarcher/sensu:0.26.5) instead of taking the latest all the time.
I also fixed the Dockerfile to apply the execute bit to plugins, as due to a mistake whilst developing this I realised it was just using the execute bit set in the repo instead which isn't immediately obvious.
At the time of creation the FROM image wasn't being tagged, now it is, so we should use the fixed tag (sstarcher/sensu:0.26.5) instead of taking the latest all the time.
As this also upgrades Sensu, I've included a new version of the check_http.rb from https://github.com/sensu-plugins/sensu-plugins-http so that the checks don't say our current method is deprecated.
I also fixed the Dockerfile to apply the execute bit to plugins, as due to a mistake whilst developing this I realised it was just using the execute bit set in the repo instead which isn't immediately obvious.