[Feature] add ability to set docker labels on worker nodes

[lionelnicolas]

Some service discovery systems are using docker labels to configure DNS, load-balancing, monitoring, metering ... (ie. dalidock , traefik ...).

This PR introduce ability to set arbitrary docker labels for worker nodes using a new --label argument to k3s create and k3d add-node.

[iwilltry42]

Hi @lionelnicolas , thank you for contributing!
This seems to be a fairly small change compared to the value it adds when combined with services like the two you mentioned 👍
Just one small note: I think that the flag name --label could be misunderstood in a way that people think it can be used to add Kubernetes node labels. Not sure if we should account for this 🤔

[lionelnicolas]

@iwilltry42 That's a good point.

I put "docker labels to every node container" in the help message for that reason, but it may not be sufficient.

We could use --docker-label instead (and drop the -l short flag).

My initial thought was that the existing --volume and --env arguments were not including docker, so I did the same for consistency.

Other suggestion, the docker-run manpage shows:

       -l, --label key=value
          Set metadata on the container (for example, --label com.example.key=value).

So maybe we could use --metadata instead of --label ?

Please tell me what you prefer, and I'll make the changes 😉

[iwilltry42]

@lionelnicolas , nah, let's stick with --label, it makes more sense in the k3d context, you're right there 👍 We won't set Kubernetes node labels anyway 👍

[iwilltry42]

I tested it locally and it works perfectly fine for me. Just left some comments that need some clarification :)
Especially it could be cool to have this feature working with our node-selectors, so that you can apply labels to selected nodes and especially also to the master node.

[lionelnicolas]

@iwilltry42 I've just pushed node-selector support 😉

--label key[=value][@node-specifier]

Node specifier values are the same as --port:

• server|master: apply docker label on server container
• workers|agents: apply docker label on worker containers
• all: apply docker label on all containers (this is the default specifier if none specified)

Note: It's not possible to combine multiple specifiers (ie. --label key=val@node1@node2@node3). To me it's clearer to repeat the argument (--label key=val@node1 --label key=val@node2 --label key=val@node3)

[iwilltry42]

LGTM! I tested many different cases any it works perfectly fine for me 👍
Only confusing case could be for someone trying to use multiple node specifiers on a label, where only the last one (correctly) would be interpreted as an actual node-specifier (as you stated in your last comment).
But this can be added to the Docs or FAQ if there is need 👍

Thanks a lot for your contribution :)