docker-cli: add docker contexts support

[will-lol]

Description

Adds docker-cli.contexts support. This allows declarative configuration of docker contexts.

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[khaneliman]

LGTM

[FriedrichAltheide]

@will-lol Each docker context must have a unique name right? If yes, I wonder if it might be better if the context name would be used as a key. E.g.:

contexts = {
   default = {
     Metadata = {
     ...
     };
     Endpoints = {
        docker = {
          Host = "unix://example2";
        };
      };
   secondContext = {
     Metadata = {
     ...
     };
     Endpoints = {
        docker = {
          Host = "unix://example2";
        };
      };
   }
};

How many options can be set, inside a context? Is the number limited? If yes, we could consider adding them all as options.

[will-lol]

Hey @FriedrichAltheide. I can't for the life of me find any schema or actual helpful documentation about docker contexts. So I'm honestly not sure what can be set now, or how that will change in the future. I think to reduce our maintenance burden, it would be better to leave as json format.
Yes the names must be unique. Having an attrset of contexts (rather than a list), and using the keys as the name makes good sense to me.

[will-lol]

@FriedrichAltheide I have switched to using an attrset as per your advice:

    programs.docker-cli.contexts = {
      colima = {
        Metadata = {
          Description = "Colima container runtime";
        };
        Endpoints.docker.Host = "unix://${config.home.homeDirectory}/.colima/default/docker.sock";
      };
    };

It supports the Name attribute as well if it is specified.

[khaneliman]

sorry, can you rebase to fix ci issue