opts: ListOpts: implement cobra.SliceValue to fix shell completion

[thaJeztah]

• relates to completion: improve detection for flags that accept multiple values spf13/cobra#2210

Cobra's shell completion has specific rules to decide whether a flag can be accepted multiple times. If a flag does not meet that rule, it only completes the flag name once; some of those rules depend on the "type" of the option to end with "Array" or "Slice", which most of our options don't.

Starting with Cobra 1.9, it also checks whether an option implements the cobra.SliceValue interface (see spf13/cobra 2210).

This patch implements the cobra.SliceValue interface on ListOpts, so that these options can be completed multiple times.

In a follow-up, we can update our code to replace our uses of GetAll(), which is identical with the GetSlice() method, and potentially deprecate the old method.

Before this patch, ListOpts would only be completed once when completing flag names. For example, the following would show the --label flag the first time, but omit it if a --label flag was already set;

docker run--l<TAB>
--label                  (Set meta data on a container)  --link-local-ip  (Container IPv4/IPv6 link-local addresses)
--label-file  (Read in a line delimited file of labels)  --log-driver             (Logging driver for the container)
--link                  (Add link to another container)  --log-opt                              (Log driver options)

docker run --label hello --l<TAB>
--label-file  (Read in a line delimited file of labels)  --link-local-ip  (Container IPv4/IPv6 link-local addresses)  --log-opt  (Log driver options)
--link                  (Add link to another container)  --log-driver             (Logging driver for the container)

With this patch, the completion script correctly identifies the --label flag to be accepted multiple times, and also completes it when already set;

docker run --label hello --l<TAB>
--label                  (Set meta data on a container)  --link-local-ip  (Container IPv4/IPv6 link-local addresses)
--label-file  (Read in a line delimited file of labels)  --log-driver             (Logging driver for the container)
--link                  (Add link to another container)  --log-opt                              (Log driver options)

- Human readable description for the release notes

Fix shell-completion to only complete some flags once, even though they can be set multiple times.

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 58.90%. Comparing base (61c6818) to head (572e3f1).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6030   +/-   ##
=======================================
  Coverage   58.89%   58.90%           
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  Files         358      358           
  Lines       29962    29964    +2     
=======================================
+ Hits        17647    17649    +2     
  Misses      11334    11334           
  Partials      981      981           

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[robmry]

A DeepEqual might make this easier to read, and avoid the nolint? The is is already imported anyway. I've not tried it, but something like ...

assert.Check(t, is.DeepEqual(o.GetAll(), []string{"bar", "bar"}))

[thaJeztah]

Ah, yes, I want to have a look at this file, as there's more to fix in that area, so for now I chose to "mostly" keep it as-is.

Will likely do some follow-ups to modernise tests in this package a bit.