Improve Dockerman icon caching.

[dcflachs]

In 6.10.1+ the dockerman icon caching mechanism does not work correctly for icons specified by the net.unraid.docker.icon container label on containers without a dockerman template. For containers created with this label outside of dockerman the caching mechanism initially downloads and displays the correct icon. If the icon url is changed however the new icon is never downloaded because the old icon remains cached and there is no mechanism in place for invalidating the icon in the absence of a dockerman template. To fix this i propose the following changes to dockerman.

[dcflachs]

My unRAID server has ~42 containers running with about ~30 unique icons. In my testing the changes to the caching mechanism do not seem to greatly effect the execution time of the getAllInfo function.

[dcflachs]

Any chance you would have time to look this pull request over and provide feedback? @limetech @bergware

[teambvd]

It'd be great to get some eyes on this whenever someone has time - I've got between 50 and 60 containers on each of my unraid servers, and loading the docker tab's gotten to feeling pretty clunky at this point, so any performance improvement would be great!

[dcflachs]

Rebased changes onto latest rev

[dcflachs]

I would love to get some feedback on this issue and proposed fix.