add optional image tag filter

To be able to do finegrained updates to container running a specific tag of an image i added this.

closes #1932

I originally wrote the code for the ?image parameter in #1289 and made it ignore the specific tag. To be honest, I forgot why I did it like that, and it's now causing problems for me.

What's the status on this? It looks good to me.

I am using this for a few months in production and it works.

I noticed this project seems dead; I'm examining forks by nicholas-fedor and beatkind to see if this can be incorporated there.

@BenjaminFaal are you hosting your fork image somewhere? If so, I think I'll just use yours.