add stack update checks

[ashishpandey]

Up until unraid 6.x, DockerMan would check all containers for updates. This used to serve as a good indicator which stacks need an update, and there was even an open PR to keep dockerman in sync with any applied updates - #27

With unraid 7.x, Dockerman does not check compose managed containers, so above PR is not needed. But there is also now no good means to check containers managed by this plugin for updates

Should be add a way for this plugin to manage its own stack update indicator, now that dockerman does not overlap?

[dcflachs]

I dont plan on implementing this. I consider update tracking beyond the scope of this plugin.